As Bongino has said the SS was duty bound to clear the threat from the area before Trump was allowed on stage. How was it that they let Crooks wander around without confronting him, detaining him for the duration of the time Trump was on stage, and/or having someone at least shadow Crooks on the grounds? What did they already have on Crooks that caused them to believe he was a person of interest?
There is a report from a neighbor of similar age that Crooks and some kids Crooks hung around with had made threats at school.
If that is true, and the threats were reported, then he was probably on a watch list.