I first played skullgirls when it launched on 360. The only other fighting game I had played was a little bit of street fighter 4. After playing through the tutorial and learning a simple bnb in training, I started on story mode on sleep walk. After finishing it, I upped the difficulty and played the next one. I wanted to challenge myself and get better at the game while going through the story. However, when I got to around normal difficulty, it took me twenty minutes to play most of the story, and over an hour just to beat marie. After maybe ten hours with the game, nightmare mode was just the right level for me, but I had to keep playing stories on an easy difficulty if I wanted to see the ending.
Casual players who buy the game want a challenge -- or at least, feel like they're being challenged, and be able overcome it. If they didn't, they would just watch playthroughs on youtube. IIRC, marie did get nerfed a little since launch, but she's still disproportionately hard. Skills you learn against marie don't really transfer to the rest of the game. Casual players won't get that she's supposed to be crazy and hard, in tradition with older fighting games. Countless people on the steam forums made rage quit posts about marie. IMO, she should be the easiest fight in story mode. She could still be hard in arcade, but really, WHO wants to be stuck on marie at the end of the story?