Edels Complete Tales - why are volumes 9-12 so hard to find?

I have volumes 1-8 of the Edels Complete Tales and despite years of searching, I've found hardly any of available second hand copies of volumes 9-12 whereas volumes 1-8 are plentiful. Just look on abebooks - many of 1-8 but no volumes 9-12. It's been that way for years. I was wondering why this was - any ideas? I know it's possible to buy compete 1-12 sets but I already have 1-8 and I am now intrigued as to why there is such a disparity in availability?