Because part of the penalty for defensive holding is an automatic 1st down. Turning a 3rd and goal into a 1st and goal isn’t good. This play was a unique case where a hold creates exactly the desired result, punter doesn’t get hit, time runs off the clock, and they get the safety to move the kick to the 20 instead of the 8.
OK. I thought you were talking about the fourth down play.
I mean the Niners’ fourth down play.