There are often competing events. There are two ongoing right now.
Much of the rest about security has been discussed. Shooters look for gaps.
Outdoor venues have more gaps. (No doors) I believe Trump's events have been indoors since.
Shooters simply go out with a gun, waiting for a gap? Its a theory.
What I would like to see is where each of this list would fit into odds. Indoor vs outdoor chances. (in theory there would be no way to get indoors without being scanned, but is this true?)
Counter Sniper or security on that particular roof vs no one on roof.
No visible security on roof vs hidden security on roof. (how would the assassin know there was no one in one of those windows? Can you see inside from below?)
If there had been more security (hadn't needed to divide it) would it have even mattered?
Was there any combination of things that tipped the scale? If there had been some kind of communication device that all the security ( local and SS) had access to and paid attention to, would that be a 70% chance it would have stopped it? If there had been someone looking out that window of the higher section, would that have been a 89% chance they could have stopped it? If there were someone on the roof, would that be a 95% chance it would have stopped it?
There are ways to get statistics on these things, but I am unfamiliar with how to do it. Maybe there isn't enough information in the public to do so yet. But someone should be figuring out if there was anything or combination of things that could have stopped the sequence of events. The answer might be as simple as "It was outdoors", except as we all know RFK was assassinated in the kitchen of his speech venue. So shortcuts and kitchens weren't a good idea either and he was told so and did it anyway.
Things only look like 100% safe if no one gets poisoned or shot at or killed, etc for the event. It doesn't mean they are safe and you can't learn something from what went wrong elsewhere. Quite honestly the most dangerous place seems to be between wherever you gave your speech and getting to the car.