Because it's offensive/disrespectful.
It'd be like if I said I found all black people funny. The idea of someone having black skin, it's funny. Or saying I found all gay people funny. Guys kissing guys? It's hilarious.
Only worse. Because religion is something you believe in and love deeply within yourself. For someone to come around and be little it so quickly and so thoughtlessly, is like a slap in the face.
And I'm not Christian. As it so happens, I've debated four a good 6 hours in the past 24 hours over the legitimacy of Christianity, and why I don't personally believe in it. But through all of that I did it in a respectful, intelligent manner.
Surprisingly.