Why didn’t Henry IV respond to his excommunication with the arrest or murder of Pope Gregory VII in 1076, and why Philip IV

Why didn’t Henry IV respond to his excommunication with the arrest or murder of Pope Gregory VII in 1076, and why Philip IV managed to arrest Pope Boniface VIII in 1303?

Henry IV, apparently, could have sent hired assassins to Pope Gregory VII, but then all European rulers would have taken up arms against the murderer – “Antichrist”, and his reign would have ended with his life. Philip IV relied on the organized church of France, which did not want to obey Rome and was ready to forgive its king for everything except the murder of the pope. In addition, it was known that Boniface VIII himself poisoned his predecessor on the throne.