So, 12 years ago, I invented Pingback. Sorry. Pingback uses XMLRPC because that was the cool thing at the time and because I was young and foolish and had a peanut for a brain. Ian Hickson took my vague spec and kicked it in the arse to make it a proper spec, and it got adopted really quickly. Back then, when the world was young, everything got adopted really quickly; there was so much new ground to be broken that coming up with a reasonable way to do something (permalinks, RSS autodiscovery, pingbacks) meant that it got picked up by everybody.