Most likely, the high profile assassinations you recall (JFK, RFK, MLK, Charlie Kirk) as well as high profile attempts (Ford, Reagan, Trump) happen in the presence of a large number of witnesses because it is precisely those times when the target is among a large number of people that a would-be assassin has the opportunity to get close enough to his target to perform the deed.
I too thought this was the obvious - and correct - answer, but didn’t want to spoil the “ah-ha” moment for the OP.