I have to say I am NOT surprised that Ravenloft and Dark Sun are so popular campaign setting (I was inspired to do Obsidian Apocalypse because of that same information). But, I am surprised there we SO MANY Spelljammer and Planescape fans out there. While they are considered to be "dead" niche categories, the interest is still strong.
WotC threw so many awesome settings under the bus with 3rd edition. When Spelljammer came out it was a breath of fresh air in an atmosphere polluted with Middle-Earth wannabe #TLA-17839-h. I was ready to throw money at a 3rd edition Spelljammer but WotC didn't want it so I moved on. I would have done the same for a 3rd edition Planescape but, agian, WotC didn't want my money. I've gamed extensively in the Forgotten Realms since 1st edition but stopped buying the materials after 2nd edition since all my friends already had the books anyway. I stopped buying anything out of the basic core books with 3rd edition since my favorite settings were declared "dead." I stopped playing D&D with 4th edition and I wouldn't give WotC a bent penny after the atrocity that is the post-3rd edition Forgotten Realms.
Now I throw my money at Paizo and Pathfinder. I love Golarion and can't wait to adventure there! In the meantime, I am combining the pulp, noir and advanced magitech of Eberron with the sailing ships in space of Spelljammer. Magepunk in Spaaaaace!
One day I'll work on updating Forgotten Realms to Pathfinder so I can play there again.
TL;DR - Those setting were only "dead" to WotC. People have been screaming for those settings to be updated to the last 3 editions of D&D.