You boys seem to forget that not only is the Pope the head of the Roman Catholic Church, he's also a head of state. Put those two together and you get invited to the White House and the U.N. when you come to the USA.
It's a common courtesy for a traveling head of state, no matter how far removed real power, to make an official visit We have to put up loopy old Queen Elizabeth and her dingbat spawn, a truly inconsequential bunch, making official visits and snarling traffic when they travel here. The Pope carries a boatload more heft and influence worldwide than the above mentioned Royal Family.
Finally, it's a couple of months away from an election year. No politician in their right mind is going to piss off nearly one hundred million potential Catholic voters by telling the Pope to take a hike.