I couldn't tell you why people are so repulsed by genre westerns. Something about it doesn't quite click in the mind. Maybe it's an American thing. In any case, I saw Wild Wild West when it first came out, and got exactly what I was promised. Ridiculous history. Steampunk machines. Salma Hayak's corset filled to bursting. Bai Ling's thong (If you're into that, I guess).
I think the reason this movie, in particular, suffers is because it works the race angle too much. Hardy har, a black cowboy. Well, news flash: Barry Sonnenfeld may be Jewish but he's no Mel Brooks. For that matter, Will Smith is too wholesome to be playing Cleavon Little. The script takes itself to seriously at times, and I honestly don't think the players involved are subtle enough to handle it. Maybe they should have stuck to fat women/dick/crossdresser jokes.
Now that time has distanced us from the film's wasted budget, this is an enjoyable piece of schlock. As often happens, people tend to focus too much on nitpicks like Kevin Kline's impression of President Grant, and not at larger problems.