Nothing really changes with time. The fact that the two of them are now fully-fledged demi-goddesses changes little about their essential personalities. Celestia, Luna and Twilight are pretty much addicted to extra-strength Tylenol nowadays thanks to the reports coming in about those two!
"Sir, why did the two Princesses' daughters start at the bottom? I mean, surely they could have demanded to be made Captains right out rather than start out as Private soldiers!"
"Lieutenant, the better you get to know those two, the better you'll understand the answer to the question. Let's just say that neither of them are the sort to let the accidents of their births let them take short cuts."
Both of them have their personal reason for not becoming princesses.
Nyx still carries in her the guilt of her actions as Nightmare Moon described in Past Sins which also involved almost executing her mother. With that trauma in mind she never wants to become a princess even though she grew to love Celestia and Luna over the years. She was at one time even Celestia's apprentice until she found out that she was groomed to be a princess. In a friendly gesture she refused the offer to be crowned as co-ruling princess of Equestria and also end her apprentice ship with Celestia. Celestia accepted Nyx wishes to live as a commoner but advised to remain close to her since being an alicorn is still an amazingly valuable asset for Equestria. Celestia advised Nyx to join the prestigious and highly elite Canterlot Special Air Service. A pegasi based special forces wing of the Canterlot Airforce. Being a member of a special forces league would improve her already impressive physical ability even more. (Having Twilight as her mother she's already a stellar mage) She would be put very close in the chain of command with Luna who oversees Equestrian defense forces.
Scootaloo has another reason for not becoming member of the royal family. She felt rejected and betrayed by her mother Celestia for being left an orphan in Manehatten. Even worse she found out that Celestia selectively and meticulously wiped her memory clean of her daughter.
When Scootaloo first found out about this she was 15 years old and got her horn and cutiemark in one night. Of course Celestia didn't know she had a daughter because her lover (Scoots father) to help undo the amnesia with a magical keyword spell wasn't there anymore. She and Luna decided to take alicorn Scoot to caterlot castle for closer observation since alicorns just to spontaneously appear. At first they thought it was another Nyx (Past Sins) incident but this time a mare modeled after Celestia. So when Twilight hinted that Scoot maybe Celestia's daughter since she saw Celestia one evening 15 years ago with a distended belly and then realizing that Celestia barely has any memory of that year she decided with the help of Luna to delve into her head to see what happened.
Luna was shocked to find out that there were unmistakable traces of Celestia's magic that had to do with this selective amnesia. So each memory phase has to be selectively opened one by one. It was around this phase that Celestia had a very fractured view of what happened that year since each memory phase was opened in non chronological order.
When Scootaloo found out about Celestia's memory spell and the truth about her being Celestia's daughter she was pissed beyond measure. Scootaloo had a harsh and traumatizing orphaned filly hood in Manehatten before she moved to Ponyville. So she felt rejected and a Celestia's bastard child that was rid off to save shame to the royal family. She turned her back to Celestia and moved back to ponyville vowing to live the rest of her life as a pegasus pony. She even at one point attempted to saw her horn off but Twilight intervened.
So she grew up to adulthood and climbing the ranks without using any magic at all. First the Wonderbolts as Rainbow Dash's wingmare but was quickly transferred to the Canterlot Special Air Service to work alongside Nyx because she was deemed too good and too hard to keep up with as a Wonderbolt.
So there you have two beautiful Alicorn mares who aren't princesses but choose to serve Ponies at the bottom of the social ladder level.
Yep. Celestia's biological Daughter. Her real name is Leane (Daughter of the Sun). Though at this time she doesn't want to have anything to do with her mother. Due to the fact that she found out that Celestia wiped out her own Memory of her daughter down to her subconscious. What Scootaloo didn't know at the time is that Celestia did that to hide her daughter from Nightmare Moon who has the ability to read dreams (just like Luna). The last thing Celestia wants is to let Nightmare Moon get her hooves on her daughter. During her unplanned pregnancy she used an illusion to look slim in her later stages of Pregnancy. Only Twilight as a filly at the time caught her off guard and got a glimpse of Celestia's distended belly. To make the plan foolproof Celestia entrusted her newborn daughter (She gave birth in the wilderness BTW) to her Stallion lover and Scootaloo's father Tempest. A mage who was a pegasus pony endowed with magical powers and powers over the weather. This is an astronomically rare Pony. A rogue at first he was turned to good by the effort of Celestia and ended up falling in love with each other. Their relation ship was kept a strict secret and kept on meeting each other deep in the Ever Free Forest. The plan was that after the Nightmare Moon threat receded that he would come out of hiding and with use of a simple spell and a key chant Celestia's memories of Scootaloo would be restored. Unfortunately Tempest didn't reckon on his arch enemy during his rogue years to make a comeback. To protect his daughter from this unforeseen threat he put a spell on her to delay her magical development as much as possible like for example delay her cutie mark and delay the growth of her horn. After having charmed his newborn daughter he hurriedly left Scootaloo in an orphanage and flew on to face his old Nemesis. He never returned for his daughter and his Fate is still uncertain.
And even then she has problems finding a date. She has dragon eyes, fangs like a predator and she's physically slightly bigger than Princess Celestia. Plus she is often seen as Nightmare Moon. Who would want to date that.
Scoots is literally an Ugly Duckling scenario. Starting from an orphan raised from the streets and often mocked blank flank (only really loved by few) thrusted to Princess.
Later on as the rather disturbing and tragic truth is unfolded (how she ended up in an orphanage) Scoots eventually turns her back on royalty and grew to despise her horn (and Celestia along with it) wishing she was just a humble pegasus pony. Scoot only was aware of part of the truth at the time. She also refuses to use alicorn magic thinking that she survived her fillyhood without it in the first place plus she doesn't want to do anything that makes her think about Celestia. She rejected the invitation to be tutored in the prestigious royal Canterlot academy of gifted ponies (The same school Twilght was in and Nyx is attending) partially because Celestia also teaches there and she wants to avoid. As rumor spread that she was an "illegitimate" foal of the Princess of the sun didn't make the remainder of Scoots high school years any better. She was feared by the other students because of her being an alicorn. After highschool graduation she followed in the hoofsteps of her idol Rainbow Dash to become a member of the Wonderbolts even becoming a wingmare of Rainbow Dash herself. After performing some impressive record breaking feats (like reaching mach one going vertically up and reaching mach 3 in a dive leveling a large chunk of the ever free forest with a massive triple sonic rainboom. She was unceremoniously discharged from the Wonderbolts organization for being "too good to be kept up with in team flying, to the point of undermining the morals and self esteem of the other Wonderbolts" This move disgusted Rainbow Dash and she was about to quit the Wonderbolts outright in protest when Scootaloo stopped her. Scootaloo later under invitation of Nyx enrolled in the Special Forces air guards. A group of hoof picked specially talented and specially trained military Pegasi whose flying abilities put even that of the Wonderbolts to shame though there is no glory since the majority of their missions are classified. Scootaloo was highly valued for her flying talents. She is seen as equal and not like a Princess and even climbed their ranks by not using magic She and Nyx (Who's a part timer under special order of Princess Celestia) form group 7 along with the best Pegasi soldiers, and tackle the most dangerous air based threats.