Because he respected floyd and was scared. Hes not that way in MMA. Plus he doesn't have the masses on his side fighting Floyd, so his words weren't as strong, like in MMA. Plus he has no experience or highlights to fall back on in boxing and prove his shit talk, so it seemed more like a dog chasing a car.
In MMA hes backed everything up, had the crowds on his side, and has proven hes great. He couldn't do any of that fighting floyd in a boxing match. Because that's what floyd is.