Probably because, while unintended, some found it offensive, and he didn't mean to offend the religious.
Personally, I think it was meant to be a response to the Pope, for inviting himself into our political governance with disapproval, while embracing Islam, and telling Christians, "not to fear Islam, but treat it with dialog and respect", while Muslims kill Christians all over the globe.
The Pope's borders aren't open. He has his gated community and we have ours.
There used to be a saying,
Only believe half of what you see, and none of what you hear. In our time, with all the technology, we can't believe
any of it.