They don't like the competition. They like to use their majority position in the public and government to give their religion special standing and exclude others.
Ah, that's why the 80-90% of us who are Christians here in the USA have wiped out all the other houses of faith and kept Jews, Muslims and Mormons from serving in government. Oh, wait...
And in the case of this specific incident, that's why the guy went after the Festivus Pole and Menorah...oh, wait...
What would people like you do if you didn't have Christianity around to whine about?