Utah has what they call "constitutional carry." Extremely permissive gun laws. I'd bet there were several people carrying concealed in that crowd, not counting security and police.
Regardless of where you stand on the subject of concealed carry, I don't think its controversial to say we shouldn't be encouraging untrained/unvetted folks to go seek out would-be assassins before they have demonstrated themselves to be a danger. That's exactly how "armed security" shot and killed an actual bystander at the Salt Lake City 50501 demonstration earlier this year.
Even so, most folks who carry prefer concealed carry for tactical reasons, one of which being that unless you have your rifle in a ready position, its not very useful in a self-defense situation, and simply marks you as "shoot this one first". And it turns out that walking around with a rifle in a ready position is generally perceived as aggressive, regardless of actual intent, even by those comfortable with firearms (consider a police officer approaching with a holstered weapon vs one in their hand).
So in the context of this shot, it ought to be relatively easy to pick out the shooter in the moment, the problem is that a ~200m radius around the tent where Kirk was speaking covers a lot of territory, and that's a lot of ground to cover effectively without obviously interfering with students' free movement about their college campus.