The rest of it is due to all of the bullying and teasing she’s gotten because of her wings from both her friends and the bullies at her school, and possibly the lack of any stronger parental figures to boost her confidence when she was growing up. But this is why Rainbow is a good role model for her, because the filly knows what it is like to be made fun of for her flying skills- Rainbow Crash, and doesn’t let anything hold her back from her dream of being a Wonderbolt. Basically, the only thing that is truly holding Scootaloo back from being able to fly is her self-confidence.
The only time she’s ever been teased was in “Flight to the Finish”, when Diamond Tiara was trying to demoralize her so as to sabotage the CMC’s contest entry and carry the Ponyville flag herself. And at the end, Rainbow Dash helped her get over that (though the episode’s dialog appeared to actively avoid answering the “disability” versus “late bloomer” question). So if it were a matter of self-confidence as well as training, Rainbow would certainly be qualified to help her.
And for what it’s worth, we don’t know how long it took Bulk Biceps to get airborne, so technically nothing we’ve seen kills the “late bloomer” interpretation. For all we know, he was several years older than Season 9 Scootaloo when he got to what we saw in “Rainbow Falls”.
Also: Ranbow Dash was there when Apple Bloom tried hang gliding back in “Call of the Cutie”, and the end of “May the Best Pet Win” implied that she came up with the idea to get tank his propeller thingy. If Scootaloo were to have been diagnosed with anything that would prevent her from ever flying naturally, Rainbow could have helped hook her up with some form of assistive technology.
Or perhaps Twilight Time should have had her studying aeronautics instead of building a unicycle, so that she might build herself something akin to Wind Sock’s glider from comic issue 81.
- She can beat her wings at an unprecedented speed
Her scooter activities make it clear that she has more than enough thrust. If she could combine that with something like Wind Sock’s glider, she could easily become an impressive flyer. With a few refinements to its design (say, switch out the DaVinci wings for a proper airfoil), she might even rival Rainbow Dash herself one day.
We have not, in ten years, seen Dash train Scootaloo once. That said, since Scootaloo is now permanently living with Auntie Loftie, if she asked for training and got some therapy, she would then be able to fly.
For that matter, we’ve also never seen any indication that she had ever been to a doctor to sort out why she’d been having trouble flying. More fuel for the Scootabuse dumpster fire.
Also: you had speculated that there was never any discussion of assistive tech because Scootaloo was exclusively interested in flying all normal-like. The sea pony part of “Surf and/or Turf” had her dive underwater and be like “So this is what it must be like to fly!”, and it formed a large part of her bias towards Seaquestria over Mt. Aris (in essence, her yearning to fly had consumed her to the point of interfering with her CMC duties - not exactly a great role model for people with disabilities, if that’s what she was supposed to be). Why, then, would she be so ecstatic about something that’s kind of similar to flying but isn’t actually flying, but have zero interest in something that’s not quite the same as normal pegasus flight but actually IS flying?
Hopefully now that the comic is no longer tied to the show’s status quo, it can give this issue the treatment it deserves. The way she was involved in issue 81 made it feel like the writers were trying to subtly say “We would LOVE to have Scootaloo partake of the painfully obvious solution to finally get flying, but our hands are tied”. And speaking of Auntie Loftie, issue 93 is supposed to focus on Scootaloo and her family. I wouldn’t expect her wing situation to be relevant to that particular plot, but it would be lovely if they could toss in at least a little hint that she isn’t being utterly neglected in that department.