Why aren't cheers seasons 9-11 available on dvd?

I have been searching 4 these boxsets 4 quit some time now. i have gone everywhere even ebay but still no luck. I've gone to many stores but they all tell me that they r not available on dvd. Well i want to know why its so hard to obtain these shows. I don't understand why they just don't make them available to fans of the show. Don't they know we want them to complete our collection. I have every season except, season 9-11. I bet you can't answer this one, cuz there is no really good answer for it.

It's marketing strategy and timing of release as an "Event." They purposely withold them so that when it comes time to release the next season, they will adertise the others as a special or make a limited edition set including all of the previous sets so that people will buy them because of their "limited edition" status. Some people buy into that and will buy these sets even though they have the other seasons. All about the money and that's what those companies are all about. Works for me since I own stock in a company that releases movies and tv shows.