The short answer is because Major League Baseball likes to study things to death and never actually make a decision. They have a chance right now to be proactive and fix a problem before a player or fan is seriously hurt or killed but they are going to wait until someone is hurt because it will be easier to get people to buy into the change.
I like the idea of changing the rules so that if a bat breaks and flies onto the field than you are out. You'll see players stop using maple bats really quickly.