That is, because it's not very funny. Out of one episode of MADtv I find myself laughing, or enjoying it maybe once, twice?
The show is intended for people who find humor in ridiculous, crude jokes. This may be sufficient to people who don't enjoy comedy with any depth, but for me it's not.
MADtv is a show I find myself watching for no reason whatsoever, or perhaps I watch it just for that one joke that makes the show memorable. Plainly, the sketches are used up. *Not to say SNL (Saturday Night Live) is much better, SNL is decent (I'd give it a 7.0 maybe), but it used to be much better, and completely worth it.
However, I've found that MADtv was always mediocre at best, and I have to say it's not lived up to its ten seasons on television.