My hypothesis is that the added muscle mass puts additional strain on ligaments, joints, and bones. Pro athletes bulk up for the power and explosion. At the same time, it's pretty well accepted that as you build muscle mass you start to lose flexibility and ROM. Muscles firing more suddenly with more force plus less flexibility probably means more injuries. In the case of slingshotting a baseball over your shoulder at 95+ mph, the weak link is almost always the ucl at the elbow.