That would likely be based on safety and logistics . Aluminum bats create higher exit ball speeds which would be dangerous , especially for pitchers.
The ball travels further off An aluminum bat which would change the game.
From a fan enjoyment perspective, maybe it would add excitement , more gappers, more home runs , who knows?
All it might take is 1 splintered bat impaling and killing an MLB player for aluminum bats to be a discussion point in MLB. They would need to change the ball though......sound familiar ?