I really don't get it either. My hypothesis is that someone didn't like the word for whatever reason, and the idea that it's inherently gross has spread like a virus. Someone tells you it's gross and gives you some gross examples, and then you think, "Oh, yeah, that is gross."
Honestly, I don't think it's even the grossest word in its semantic realm. Damp seems grosser to me, but I still don't have the same aversion to damp that some people have to moist.