Spit balls were originally banned ostensibly for batter safety. This was in an era before batting helmets and replacing the ball nearly every time it touched the ground. It’s not clear that reintroducing it would cause a safety concern, but MLB is interested in having a certain balance of defense and offense and giving the pitchers back the spitter would require some offsetting change for the offense. With the recent clampdown on PEDs, I think the balance is probably temporarily too pro-defense, so it seems unlikely that the league would be inclined to reintroduce.