The most common explanation I've heard is the cost of a season worth of wooden bats would be much greater than having a set of aluminum bats. Because the wood bats break, and the aluminum bats (very rarely) break. I haven't done the research, but that's the response I see whenever this question comes up. And I think it's one of those "what about the smaller schools" arguments. No doubt the big schools could afford an endless supply of wood bats - it's not like baseball makes a profit for the school anyway.