Because in MMA if you wear shoes you don't have the right to use your feet to kick or anything else plus MMA fights do not last as long as boxing in general as it's normally 3 rounds of 5 minutes opposite to 12 of 3 minutes, doesn't use scratch the feet in the end the same way. I think as well that the tradition is playing a role too in the whole thing, a bit like the entrance robe which is very frequent in boxing and quasi absent in MMA.