It was fear. Maybe not on the part of Trump, but it was fear. The SS doesn't give a flying fig about the safety of anyone but the protectee.
If they rushed the Stage going after Trump, the Secret Service Agents would have had a Live Fire Exercise.
By cancelling the Event, Trump saved Lives.
The Idiots who had their Lives Saved have no idea that they are still breathing thanks to Trump, the guy they hate.
That is their job.