Read an interesting article on how Tie-in books (e.g. star wars, star trek, et al) are gradually crowding out original stories.
Quite frankly I think the biggest reason this is happening is simply because there isn't that much good original sci-fi fantasy fiction being written. Ever since tolkien wrote his (good though dry) LOTR he influenced generations of fantasy writers/lovers afterward to follow the elf/dwarf route in their writing. It's been ages since I've read a good fantasy story that didn't dip into the myths of elf/dwarf/gnome. And when stories like that DO appear, readers end up loving them and so comes the sequel after sequeal after prequel.
I think really there are two complaints here - the complaint of endless sequels, where one popular novel turns into fifty - which quite frankly I rather like. If I take the time to learn about a world, it's always interesting for me to read more about it, and the complaint of tv tie-ins - which I actually understand. But the same thing applies to TV tie-ins. If a reader enjoys a book, a world - or a viewer enjoys a movie or tv show - it's always EASIER for the reader to enjoy something they already know instead of having to learn a whole new world with each novel. It's something that only sci-fi/fantasy and historical books suffer from - having to teach the reader about the world. And readers are lazy, myself included.
I don't see this as something that's ever going to change, though.