I think your misunderstanding the influance of the mainstream on TV. The Simspons might seem unfunny to you after all this time, but then agin your here on "The Escapist". Odddly I think you overestimate the everyman. Ask yourself how things like Pro-Wrestling continue on, and they have been around longer than "The Simpsons".
"The Simpsons" is one of those rare productions that hits solidly in that 'sweet spot' for the everyman. It's simple, uncomplicated, and can make your typical person chuckle and think themselves clever. It takes someone outside the mainstream to really look at it and go "you know this is really pretty insipid".
On top of that the series has become iconic. "The Simpsons" have spawned a merchandising empire and become as well known as Disney characters and the like. What's more you'll notice that "Universal Studios" has a pretty substantial ride and an area built up around it as one of their attractions to compete with Disney.
I'd imagine the show gets automatically green lit because it rakes in tons of dough. What's more it's doubtlessly seen as being "safe" territory for advertisers and has decent guaranteed viewership, and thus represents prime advertising real estate.
You really can't critique Fox for going where the money is, when most of the shows mentioned really didn't have that large of a mainstream following as far as I can tell.
"Firefly" is perhaps the one exception there, as I think that one was pretty heavily followed due to it following the the coattails of "Buffy". The issue wasn't one where I think the show got nailed because of the show, or the audience, it was because of Joss Whedon himself who was supposed to have been in fights over creative control of his own productions.
Back when this was current I remember reading allegations that the big reason why the last season of Buffy was lacking was because Jos was having the network dictate to him what he was going to do, and forcing him to insert characters for the sake of political correctness. Apparently a few of the potential slayers wound up getting forced in by network executives and he was forced to write things to give them "X" amount of screen time. The arguements being that Buffy was "too whitewashed" and they needed to put more minorities into the show. According to rumors the whole bit with the principal being a demon slayer (and the whole sub plot leading to his fight with Spike) was also the result of forced insertation. The whole season being a "design by politically correct committee" affair.
While "Firefly" was hardly whitewashed, and had exactly the people Joss wanted in it, apparently his reputation in the industry was so bad both from "Buffy" and "Angel" and he was seen as such an uncontrollable force, that he was pretty much slated for the axe, the following he could garner, and the amount of money he could make, being irrelevent since the powers that were couldn't deal with having someone involved who was getting away being bigger than the networks.
Whie Sci-Fi Channel briefly considered picking up "Angel" and claimed they wouldn't do it as a bad business move, due to having to compete with their own show's previous episodes in syndication on other networks, allegedly they didn't want to deal with him either.
That's all rumor and supposition of course, since nobody knows much about what actually happened. While the details might be off, I did definatly get the vibe that his name was mush with the networks and that is why his shows were killed.
