Cause I agree with SF that it would throw the morale out of whack.
Other teams bench underperforming players all the time. Look at the Braves!
This is one of the things that I like least about this organization. They have an unseriousness about winning. I'm not calling for punitive actions. They need to be able to explain why things are being done and the explanation for benching Span is easy, it gives us a better chance to win. The explanation for moving Span out of leadoff, his OBP is too poor, he doesn't see enough pitches...he doesn't walk enough.
This organization, Rizzo being the biggest culprit, acts like results don't matter moreso than almost every other team in baseball. If you put a player in a particular spot for a specific reason, that player has to perform or you move on. You don't drag it out and hope things work out.