They have to move him as fast as possible to the black SUV. Any further gunshots have to be absorbed by the SS officers surrounding the POTUS. Once one shooter is down, they have to move as quickly as possible to get him to safety, putting their own lives at risk if there's a second shooter. Once he's in that black SUV, he's pretty safe and they can get him out. The whole point is to get him in the vehicle and out of the area ASAP.I don’t get that either. While Trump was still on the platform in the huddle one of them said “shooter down”. It was a matter of seconds. How could he determine that there were no other shooters? You would have to be looking from an arial view to see if others were lurking around.
He's a big guy, not easy to move and not easy to cover if he refuses to duck down. I recall from watching the replay that day, that he seemed to be resisting the SS agents who were trying to push him to the car. I could see they were struggling to get him moving once he was on his feet. He kept trying to turn to the cameras and wave/fist bump or whatever. That was slowing things down quite a bit. Once they got him to the black SUV, he resisted them again, standing up to wave or do some other gesture. IIRC, one of the agents finally just shoved him into the SUV. They really struggled. It's possible Trump was still in some kind of shock where he didn't fully understand the danger and kept fighting them and slowing down to wave. Same with the shoes. Presidents most likely receive instructions on what they have to do when the SS goes into action to get them out of a dangerous situation. Trump probably knew what he was supposed to do, but was probably in some sort of shock and not comprehending.
JMO