They soften the blows too much. It might seem weird, but being knocked out cold is actually what protects your brain from damage. If you never get knocked out because your gloves are gigantic and full of padding you are more likely to take a serious amounts of hits. That's why the MMA has way fewer brain injuries than boxing. The 4 ounce gloves are ideal.

Also this is more anecdotal, but I remember reading that in boxing you can punch as hard as you can consistently because your hand won't hurt so bad from landing head punches. In MMA the less padding on the gloves means your hands will hurt way more from throwing punches which reduces your power the longer the fight goes on.