It's like the purest form of "situation comedy." In typical sitcoms the situations are relatively normal and then the humor comes from the cast basically joking their way through the situation. In Seinfeld there's no "jokes" in the typical sense, the humor is in how stupid the situations themselves are and how absurd the cast's reactions are, and the punchlines are the situations coming to a head where the characters either get comeuppance or the situation collapses under the weight of the dumb.
Also I think it was rather relatable to that generation. Like "these are terrible people you might actually run into, dealing with the stuff you deal with but badly." It's basically observational comedy acted out. It was actually conceived by Seinfeld and Larry David as "how an observational comic gets his material," where each episode would begin and end with Jerry doing stand-up material about the ridiculous shit that happened to him (Seinfeld himself famously hated that people began saying it was "a show about nothing," because it was about
that), and eventually the show outgrew that framing and ran on the momentum of its own character types and bullshit situations once they had their own kind of world built out.
tl;dr: it was Gen-X cringe humor before "cringe humor" was a thing.
Also Julia Louis-Dreyfus and Jason Alexander are
fantastic in it, which helps the appeal
a lot. For the people it does appeal to, anyway.