To be honest, the hotter bats is exactly the reason the masks are now a mainstay in the fastpitch game. I honestly don't think the masks would be as popular if the game was played with wood or basic aluminum bats.
Hotter as in a faster exit speed is a popular myth. The max exit speed if anything has gone down. What has changed is the size of the sweet spot resulting in more hard shots resulting in an increase in the number of injuries.