I’m fairly sure Human Twilight becomes an alicorn because she’s pretty much stealing Pony Twilight’s power, just like the others are empowered by the Equestrian magic she brought through.
 
Basically, pony Twilight gave the others powers when she used the elements of magic through them. Human Twilight got powers when she stole that magic from them and detonated it in her own face.
 
Sunset Shimmer is the only one who’s actually using her own innate magic.

HT: “This feels weird, walking around on four legs.”  
PT: “Don’t worry you’ll get the hang of it, might even start to like it!”  
HT: “Right…”  
PT: “Just follow my lead!”

@Background Pony #2031  
True, that’s a very good point. I retract my previous post, and unicornified Sci-Twi makes perfect sense. :D

@Yet_One_More_Idiot  
Because she’s the alternate reality version of a unicorn, which apparently matters. Fluttershy and Rainbow Dash got wings when they got magic, just because their alternate reality versions are pegasi.
 
Don’t forget that earth ponies have magic too, so her becoming an earth pony just because she lacks magic right now doesn’t make sense either.

Great art! I really want to see more of human Twi and pony Twi (either both in human or both in pony forms) together. Do we have a shipping name for them at all? :)
 
Apart from the fact that Sci-Twi took in all the Equestrian magic to become a superpower and evil alicorn/human hybrid, why should she be an alicorn when in Equestria? She doesn’t have any innate pony magic. Indeed, why should she even be a unicorn in Equestria for the same reason? I find myself imagining Sci-Twi going to Equestria and being revealed to be an Earth pone instead, that makes a bit more sense in my head. :)

They really should make out. I’ll get my camera. >:3

HT: Why do you have wings but I don’t?  
PT: The answer is quite simp… Well you see… Big crown thingy. It’s magic I don’t have to explain it.  
HT: Fair enough.

@Background Pony #CB4F  
She was only temporarily an alicorn. As far as I could tell, the horns and wings were magic constructs.

Twilight is cute all the time

Doing the right thing how again? Midnight Sparkle was depicted as an alicorn, and so have been the ponied up toys of Human Twilight.
 
However there is room for Human Twilight to grow so I can see why the unicorn form would make sense.

Nice to see the artist did the right thing and made human Twilight a unicorn.

Too much cute!!

“and now for a lesson on how to probably kiss me”