It's cool to dislike anything that's popular, which ensures boring people hate a healthy mix of the terrible and the sublime.
More specific to Seinfeld, however, is the large number of people who can't empathize with characters who aren't culturally brainwashed. The characters in Seinfeld are too overt in their personal motives for the truly deluded to identify with them.
I don't think a lot of people ever enjoyed Seinfeld. By that I mean most people said they liked it but never actually watched it or did watch it but did not understand the premise(or lack thereof).
Most likely though I'm just talking out my ass. No, I'm talking out my ass.