Because they significantly increase the length of your rifle.I love muzzle brakes, even on small caliber rifles. Why would some folks be against muzzle brakes?
Because they can change the point of impact when you take them off.
Because they direct the sound back towards the shooter and increase the decibels reaching the shooter's ears.
Because they kick up dust and debris when shooting prone.
Because it's extremely annoying to be shooting next to someone on the range who's using one.