I think it was a combo of the sponsorship ending and the chance to put an IP in its place. There were a lot of people complaining the scenes needed to be updated to more current movies, I'm not sure Disney wanted to spend the money to do that. It worked for them to put MMRR there if they weren't going to update it. They had been trying for a while to make HS more kid friendly (thus the whole Toy Story area). I don't think the violence had anything to do with it, there really wasn't that much and certainly not anything any kid wouldn't see on TV. I think it also had to do with the whole re-branding of HS from MGM and making it more current.
I wasn't a fan of the change, I've ridden MMRR a few times and find it a bit too busy and the dance scene is too jerky. I actually think GMR fit into the new branding of being inside the movies instead of seeing how they were made. I'm one of few who actually liked the old park and one of the reasons was because it wasn't as kid friendly (although my kid loved the heck out it).