Why are aluminum bats not allowed in Major League Baseball?

Pitches are thrown faster in MLB than in leagues that allow aluminum bats. In addition, balls struck off aluminum are faster, so if a pitcher were to get hit by a ball hit off aluminum, he could get seriously hurt.

Originally tradition, and maintaining a consistent equipment for a sport in which historic records are important. Advances in aluminum bat technology lead to increase injuries that are only going to increase as the ability of the hitter increases.

There are on going rules committee discussions to remove aluminum bats from division I NCAA college baseball because of the injury factor.

From this point on it was evident that the ability of manufacturers to manipulate the size and weight of baseball bats created an injury hazard and a player-development problem for collegiate-level baseball players.

...These lighter bats allow for faster bat speeds during swings that result in a greater hit-ball velocity. Because the ball exits the aluminum bat with a higher velocity than would a ball from a wooden bat, there is naturally a greater danger of injury to defensive players. "Any idiot can see that the ball jumps off an aluminum bat faster than off of a wooden bat," said Jim Morris, head baseball coach at the University of Miami.

https://www.reference.com/sports-active-lifestyle/can-t-mlb-players-use-aluminum-bats-c443401844aeef3b#

https://www.exploratorium.edu/baseball/tools_4.html

Someone would get killed. Speaking from experience here, take one off the melon. The speed of the ball is just too much to react to.