Because not everything is racist, but there always seems to be a black person saying it is racist. We just get tired of it. Heaven forbid the situation is really because we don't like that person, not the color of their skin. Just by someone saying the situation is racist, we dislike the situation and that person who said it even more. They cause racism upon themselves by saying it is racist.
If you disagree with a black person, it could be because they have a bad idea, you don't like their personality, they aren't being organized, etc. IT'S NOT FROM THEIR SKIN!