The rating has a ton to do with who the game is marketed towards, hence why halo went from an M for mature rating to T for teen. They try and reach a wider audience. It's the same reason as why there weren't any R rated superhero movies until recently. Studios didn't think there would be a big enough audience as only adults/teens would go see it. They made the game T for teen for a reason, to try and reach the widest audience there is, hence how it's marketed. It's much easier for a child to convince a parent to be a T game then a M one.