I've noticed most shows released on DVD are aired on a national network at the time their released or have a high syndication rate. so I think the companies measure the popularity of shows by this and thats why shows like Gimme a Break, The Hogan Family, Empty Nest and others aren't coming out.

I also think companies really have to spread out their releases. If you release too much too quickly, the market will be saturated and people will only buy their absolute favourites. By spacing out release dates and certain tv shows deemed as competition, it creates a better market. Hoepfully we just need to be patient.