 Until a gunman already shot him in the head. Because even if you want to claim that for some reason they couldn't confront the suspect right away or which it doesn't make any sense. Even if that were true, well, you can take Trump off the stage at the very least until you manage to get the weird guy with the range finder and the backpack off of the building. Why didn't Secret Service snipers on the roof behind Trump take out the shooter before he was able to open fire?