If the initial prize was 60 dollars and the subscription was 15 dollars/month, people payed 240 dollars for the privilege of playing said game with the content of what, maybe 2 Kotors ? That would still be double the amount of cash you would usually need to fork over for 2 brand new AAA single player RPGs.
Ok, maybe the content exceeds 2 Kotors, but it's still hefty.
And don't forget the limitations an MMO has over a "true" single player RPG: The world can't really permanently change for everyone because of your decisions, your decisions in general are "smaller", combat is often "dumbed-down"/less strategic, things need to be balanced, so there is less of the "I'm a God and I'll smite you" entics that often happen at the end of RPGs (where you get the super powerful weapons/skills/spells) and that a lot of people enjoy, etc. . .