This article has great video of the scence as law enforcement/SS prepared to escort Trump from the stage. IMO, it took a long time! There were more LE protecting the path as the vehicle drove away than there were surrounding him as he left the stage.I found a different picture of the water tower in relation to the stage and the rooftop TC shot from. You can see the legs of the tower behind the #3 photo. It appears that it is behind the stage which might not have given him a shot. The article also shows the roof, which was supposed to be unsafe. There are prominent ridges to the metal which wouldn't have been a safety hazard for the SS.
Video reveals chaotic scramble as Thomas Crooks fired at Donald Trump
Thomas Crooks, 20, launched eight rapid fire shots from his AR-15-style rifle at Donald Trump in under six seconds before snipers fired two counter shots, including the one that killed him.www.dailymail.co.uk