It’s probably not easy to take careful aim while in a large crowd without someone noticing— especially since there are skilled professionals whose only job is to watch for people taking careful aim…
Person aimed for the head, seems like he wasn't trained how to kill people.
Anyway, the person knew they would die seconds later, heavy breathing makes it is massively harder to aim when you are nervous, then it is easy to miss a moving head at 140 yards.
Rifle rounds go straight through medium vests, you need a really heavy and bulky vest to stop them, Trump wouldn't wear one of those. The typical vests you see just stop low caliber bullets like pistols, and then you hope secret service can stop anyone with a rifle from getting a shot.
The times I’ve worn vests I’ve had to have ceramic plates in for ak-47 style bullets, but I’d assume that the secret service would have something more fancy available.
There is actually now very expensive cutting edge stuff (FRAS) that’s flexible and can barely stop a 5.56 round, and i’d assume important people have whatever top secret upgrade exists for that, but that could be wrong.
That's fair, but it's only part of it. The issue of "stage fright" still exists and can't be mastered just by practicing your craft to proficiency in solitude. There are other practices, like literally going on stages (speaking, performing) and becoming used to the pressure, that would overlap.
Hopefully I'm not giving useful tips to future assassins. :P