Kyle: A goofy neighbor character can really change the dynamic of a sitcom. Family Matters was a run-of-the-mill family story that benefited from the inclusion of Steve Urkel because it gave the show a hook. An individuality. Even a reason for folks to remember it. It became a show that centered on the wacky neighbor himself, and that's a bent formula that has even been reused and adapted for shows like Home Improvement.
Meanwhile Seinfeld is a show about nothing, and that individuality or hook was not affected by the presence of Kramer. With or without him, the show would still be about nothing.
And one major thing about the wacky neighbor: There is a penchant to ruin things. If there's a vase, they will break it. If there's a meeting, they will trash it. But at least Urkel taught everyone a lesson when he wrecked things. Kramer, on the other hand, would just show up and destroy stuff, and that was that.
Further, Urkel was very well defined in his archetype. The science, the goofy dancing, the clumsy ... striking. He wore a specific hat. But Kramer was nebulous. He started somewhat sleazy, but became more legitimate when the episode demanded it (including meetings with name-brand companies about his inventions). He's a high-class vagrant, at home dumpster diving and smoking Cuban cigars and sipping scotch with the higher-ups.
Finally, Urkel is the superior wacky neighbor because he has a relationship with each of the Winslows. Carl is the father he never had, Eddie is his best friend, Laura is the love of his life. Even Richie is the only person who will look up to his goofy ass.
Meanwhile, Kramer doesn't have that strong an impression on anyone. Jerry and Kramer tolerated each other, mostly due to their living arrangements. But he really didn't have a big thing with anyone. Jerry is still sarcastic, Elaine is still horribly awkward, and George is still a terrible person. Kramer never really made them more than what they already were.