Because now every time this guy's gotta visit the United States, he's gonna get this:
And it's not like this guy had any tradition to follow. There's no previous Popes by this name: there's been a ton of Gregorys, an army of Johns, enough Benedicts to feed a breakfast buffet, a surprising number of Clements, not enough Innocents, seriously not enough Eugenes (CAREFUL WITH THAT AXE)... no, this guy's gotta go with a new name ready for mockery. He could have gone with Urban, that's hip right now, or Lucius... no, he's gotta go with a name that's gonna be on Zazzle and CafePress t-shirts with "Lighten Up" right above the "Francis".
On the other hand, it's bound to give the actor playing Francis a boost in popularity on the IMDB website.
On a serious note, I'm not sure the Catholic Church did themselves any favors promoting a Cardinal opposed to same-sex couples adopting, considering that promoting adoptions overall promotes a pro-life message.
But what do I know. I'm Unitarian: I didn't have a vote in the matter.