The same reason she didn’t do so when falling from the hot air balloon, she can’t remember to do so when frightened.

because running works just fine

Because…!… just because

probably hard to do when she’s panicked.

@monkeybarrel
 
Headcanon’d.
 
After all whose to say Equestria isn’t a D&D campaign setting? I mean Hasbro does own WotC

She was out of magicka.

:T  
as I usually say when this question is asked Twilight does not work well under pressure and forgets things

She had used her casts for the day?

Because it would be boring then.