Because he speaks their language. Of course the word fundamentalism is a slippery one, and yet today the vaguely defined group known as Fundamentalists are the Jews or Gays or Communists of yesteryear. Say anything negative you want about them, and you'll be applauded.
Now, the trick is knowing that we are all fundamentalists, and we are all liberal. There are some things we won't budge on. Say, opposing the Global Warming bandwagon. I don't see Pope Francis being very tolerant or open to dissension on that one. Likewise, we're pretty liberal about things we don't like or don't care about.
And yet, in just a couple decades, Fundamentalists, Terrorists and Nazis have become one and the same. Contrast that to only some years ago, when the liberal television show West Wing had an episode reminding viewers that fundamentalists and terrorists were not, in fact, the same (though despising fundamentalists was certainly understandable):
So with the liberal redefining of the terms to say that the vaguely defined Fundamentalism is inherently evil and wicked, and Pope Francis agreeing, rather than calling for nuance and a reminder not to assume evil is always because of 'those sinners over there', once again he repeats the words of liberalism's latest gospel. And does so with aplomb. The type of aplomb that would typically be associated with - you guessed it - your average stereotypical fundamentalist.
Note to Pope Francis: Listen to his answer in the clip when it is is mentioned that Christian fundamentalists are the equivalent of Islamic terrorism.