I like to think that the reason it's still on the air is because it deserves to be on the air.
I also like to think that a good portion of us were teenagers when the show originally aired and we either dismissed it as bad, or were unable to pick up on the subtle nature of the humor. Additionally, i like to think that it has also become more relatable. When you're young you relate to Bobby, since you wouldn't be able to relate to anyone else in that show. However, when you get older you realized that you know the majority of those characters.
For example:
We all have a paranoid friend.
Or a friend past his prime.
Or a womanizing friend who seems shallow but is actually quite deep.
Or somene who hasn't changed much since highschool.
Or a friend who is cheating on thier significant other/or who's significant other is cheating on them.
Or the Jones' and the people trying to keep up with them.
Or Some or all of the above.
Clearly a 13 year old cannot relate to the majority of the above, along with pick up on all of the innuendo or crazy adult oriented antics.
King of the Hill is quite possibly some of Mike Judge's best work, I will admit that it can get a tad stale after you've seen an episode several times, but you know what, the same goes for Family Guy.
I bet you don't like Mission Hill or Home Movies. I bet you'd complain more if it was Mission Hill/Home Movies at 9:00 instead.
Count your blessings little one, and attempt to watch the show, if you still don't like it, perhaps you should try to entertain yourself another way during that time.
Oh, a signature, neat. If you're reading this, you're reading something that was written a long long time ago. That or i decided to lurk today.