Diablo II is still extremely popular. People make a lot of money out of selling items for it over e-bay, haha. Besides, if they hadn't released WoW they would be releasing Warcraft 4 instead and everyone knows it.
Why does WoW have to have a monthly fee @@@ Well, it's obvious - 'cause they make loads from it, but still. Almost everyone would have it if there was no monthly fee I recon.