According to Breaking Points, text messages from local law enforcement show they were aware and tracking the shooter and knew he was carrying a rangefinder, up to 1.5hrs before he shot Trump.
Local police photographed the suspect and suggested notifying SS Snipers to "look out" and sending in someone to "check it out".
The Secret Service and those up in the chain of command appear to have ignored the warnings and their requests.
Video here: https://youtu.be/6VIAGBfnNvQ
Ignore what a lot of them are saying now. Each group are trying to shave the blame off to the others. Reality is the Secret Service did not do their job to defend the President (both their job at providing active defense and department management of the other groups involved.) The “why” only matters now if they all did it on purpose. That blame is now gone with the head of the Secret Service resigned. She took the full blunt as a good little puppet. A new one will replace her and it’ll be up to Trump to use it or to just stick with his own team.