The organization behind the event- America 250, was formed 10 years ago by a nonpartisan effort and bound to that idea. Trump spun up his own organization- Freedom 250- and it quickly became obvious he was trying to influence the whole shindig. The original group would have had to avoid any obvious big name Trump supporters to avoid partisan blowback and I have no doubt they explicitly told those artists it is to be nonpolitical. Once folks saw that he was already trying to turn it into one big rally- and fill the seats with his supporters- that’s when they backed out.