In this case i would say because they are a bunch of democrat clowns. So a bit like the other guy said, gang colors.
That's a good point. I was semi joking in what i said. If i had to give a serious answer i would say most are blue because blue is seen as a calm, relaxing color so the environment is more inviting. If they were red or most other bright colors it would seem very aggressive in my opinion and the message maybe not as well received.