The Pope and the President did not always agree on every issue (death penalty comes to mind), so it was classy of the President to so honor the Pope. Bravo to him.
As to why other Presidents didn't attend papal funerals -- well, probably a lot of reasons. Probably worried about offending the separation of church and state crowd. Im' thinking it's more likely that none of Pope John Paul II's predecessors had such an impact. I can remember when popes often just stayed in their little "ivory tower" in Rome and made all these proclamations without having a clue as to the needs of the peope. This Pope reached out to everyone of all faiths (or no faith) all over the world.
When John Paul I died in 1978-9, Jimmah, classless, Carter sent his mother Miss Lillian.....who wisely stated one time "I should'a stayed a Virgin."