Bella Thorne is a divisive celebrity because she marches by the beat of her own drum. If you follow her on social media then you know how outspoken she can be on just about everything from her sexuality, to her past, to her style. For that reason, she has a lot of haters, but it also means she has a lot of fans. As far as young people go, she has that market down.
She's able to play "the bitch" just as well as she can play "the girl next door," so she has a wide range of acting skills to tap into. Why not cast this explosive young star? She's a little too young for the role, and as many acting skills as she has in her arsenal, superhero is one that she is untested in.