Of course they could, but typically most events aren't known about so far in advance to the public. The White House correspondence dinner is always the last saturday of April at the same location.

This gives people like the cartel tons of time to get blueprints, logistics, all the things needed.

Meanwhile if there's an event he's speaking at and they don't know that long in advance, nor know it well enough that they can make quick adjustments when they find the way things have shifted around, as far as security, they have a much harder job ahead for themselves.

Most "will be here in the same spot every year" sort of deals happen at the white house grounds itself.

The other places where he speaks not in a predictable place every time and place, they have more logistics they have to figure out, thus giving an advantage to the secret service who can figure out a location far sooner than the cartel can. By the time the cartel learns about a place Trump will be at, the Secret Service has already analyzed maps, strategy points, etc.

With the Correspondence dinner, the cartel has the same time advantage as the secret service does, meaning the risk is far greater.

When Trump gives the state of the union, again it's at the white house meaning you have to be downright suicidal to attempt anything there. It's like trying to steal from Fort Knox compared to the local bank.