In MMA there are also other ways to attack somebody and win than just punching them in the face which helps cut down on serious or long term conditions that are debilitating or end up fatal. You can submit somebody and they are probably not going to suffer any permanent damage. Even in the rare occasion where somebody doesn’t submit and something like their arm gets snapped for example, it will be painful, but they aren't going to die from it. With boxing there are too many times where they person is kept up and they are literally punched in the face dozens of times after they have already taken too much damage and shouldn't be fighting anymore and it ends up messing them up. The ability to take somebody to the floor also helps take away the dozens of the blows to the head that boxing would have in that situation.