Kind of hard to race to get somebody down when he is already on the roof, and armed, while the guys who might be willing to try are on the ground and unarmed. And I didn't see any ladder in that video to the roof either, so climbing up there would presumably have taken enough time that he could have taken several shots before anybody did get to him.

They did try to point him out to the presumably armed police (don't know if it was just one, or several officers, but there supposedly was at least one) nearby, which sounds the sensible choice to me as the police at least should have been able to both divert his attention and notify the SS near Trump if they had, I don't know, maybe try to shoot at the shooter from the ground level? Except the police did nothing.