 As Sam Andrews points out, the shooters just didn't know enough about wind correction. And oh yeah, don't forget the other obvious agenda too. And I will suggest that the reason for that is, rather than using a .308 or the .300 wind mag or any of the other cartridges that would have in fact done the job, they had a political objective as well, not just to kill Trump, but to make sure that in the process they could further demonize the dreaded AR-15, which is just plain too light of a bullet, even at this range.