Because he also said that same-sex marriage "disfigures God's plan for creation."
As you know, these realities are increasingly under attack from powerful forces which threaten to disfigure God’s plan for creation and betray the very values which have inspired and shaped all that is best in your culture. -- Pope Francis, 2013 The Advocate Man of the Year
Keep in mind that gay and lesbian Christians have been metaphorically sucking this guy's dick for over a year now for a couple of speeches about homosexuality in which he literally said nothing new or useful. His statements were entirely in-line with already-present church doctrine which paints queer people as sinners who do not deserve our relationships fully recognized, who are inherently unfit parents (one of Francis's pet issues pre-papacy was fighting against same-sex couples' right to adopt children), who are "called by God" to be lifelong celibates (hence any iota of support for gay priests). The only thing pro-queer about Francis is that he is against murdering us and occasionally compromises on key issues in order to avoid accepting our full humanity.
But let's back up here. I didn't put in my title "homophobic bigot" like I did for my last two Pope Francis essays (listed below) because his problems go way beyond his stance on homosexuality, and it starts with this doozy of a position on family planning. Keep in mind that Francis--like the Church in general--opposes contraception, hands-down. The only contraception methods the Church supports are ones that basically involve guessing whether or not a woman is fertile on that particular day. Whether they're effective or not depends on the method used and how accurately those methods are followed, but regardless, a very observant Catholic has very few options for birth control and is required to just grin and bear the possibility of getting pregnant whenever they have sex. For those who can't properly get the hang of Church approved methods, there is no backup birth control (no morning after pill if the condom fails, no hormonal birth control if you happen to have sex at a fertile time, no abortion if you get pregnant anyway). You're supposed to just accept that God might want you to have a baby like it's no big thing. So here's a guy who fully supports that kind of sexual environment in his religion, and he can still tell a woman with seven kids that she's irresponsible?
It frustrates me to have to say again, but Pope Francis just isn't that much of a breath of fresh air. He's championed a select few progressive causes--things that don't challenge Church doctrine in any useful manner--and has continued to espouse staunchly conservative, bigoted beliefs elsewhere.
* As an anti-marriage queer it might seem odd that I would be so irritated at the support of people who are against same-sex marriage. The reality is that there is a huge difference between being against marriage as an institution and being against marriage only for same-sex couples or trans folk. More can be read here.
** For an earlier take on his popishness you can check out my earlier essays, "Pope Francis is a Homophobic Bigot" and "Pope Francis is STILL a Homophobic Bigot."