I agree with everyone whose frustrated with this show no longer being released. However, I disagree with some of the "reasons" that it is supposedly not selling:
Picture Quality: Yes, this show does indeed look poor on DVD. However, it did not look great to begin with. It wasn't meant to. The "look" of the show is supposed to represent the lower middle class life of the characters. It was designed that way. This is not the day-glo world of Friends. I'm not sure how far back they went to makes these transfers. Maybe not far enough. But AITF will never have a pristine presentation on any format. Plus, I don't think the picture quality has anything remotely to do with the DVD's poor sales. Customers seeing these releases in a store would have no idea about the quality level. Many would not even think about it.
Promotion: Well, you could use this excuse for a LOT of DVD releases. It's true that this show could and should have gotten a LOT more promotion on TV and in print. Perhaps also having the remaining cast promote the DVD's. However, we live in the age of the internet. Anyone wanting this show on DVD can easily find it and get it with little trouble. It's not exactly hiding in a dusty storage room down at your local Barnes & Noble. Fans know about these releases. They're just not buying them enough.
Extras: Certainly with a show that's this legendary you would want there to be some extras. Retrospectives, commentaries, something. But there are TONS of classic shows that don't get any of this either. People are not refusing to buy these discs simply because there aren't any extras.
Everything comes down to sales, folks. If people are not buying AITF in high enough numbers then they simply aren't. Too many older sitcoms don't sell well because older customers are not buying DVD's as much as young customers. And younger customers aren't as aware of shows like this because they are not still being played to death like The Brady Bunch & Full House. This is also an intelligent, adult show. That's often the kiss of death.