Even putting aside my own opinions on Chastain's acting, in that I feel like she's just okay, let's be realistic for a second. Even putting aside the matter of filmography, look at the rest of the cast and how they've gone about adapting some of these characters.
I'm sure most of us didn't expect Reeves to jump outside the usual pool of white actors for Gordon. I'm sure a bunch of us expected them to cast an actual heavyset, overweight actor for Penguin instead of someone like Colin Farrell under a buttload of prosthetics. And I'm sure most fans thought they'd cast a woman who was naturally on the bulkier side for Sofia Falcone.
Be honest, aside from thinking she's a good actress, most people really only want Chastain for Ivy because she has red hair. I'm sure that if Ivy is in this, it's very possible they won't just cast a super duper famous white redhead and be done with it.