Modern MMORPGs are huge beasts that allow for a large variety of gameplay modes. You can PvE solo, in small groups, or in raid groups. Or you can PvP solo, in pseudo-solo automated groups, in small arena teams, or in huge guild versus guild battles. The advantage of offering so many different ways to play are obvious, players can choose what to do depending on their mood and how much time they have available. But on the other hand the various game modes cause problems for each other. Especially class balance seems to be difficult: some classes are good soloing but not so desirable in a group, others are the other way round. Class abilities get nerfed to achieve balance in PvP, although that ability might have been perfectly balanced for PvE, and is now underpowered. Abilities and item stats like aggro management end up being useless in solo and PvP play. Gear progression from PvE doesn't mix very well with PvP, which is supposed to be more skill-based. Meanwhile making all these huge games becomes increasingly expensive. So why not make games that only have one of these parts?
When Age of Conan and Warhammer Online were announced, it appeared as if these games were mainly about PvP. But either that was just a marketing trick to differentiate these games from WoW, whose PvP isn't its strongest point, or the developers changed their minds: From all what I hear both AoC and WAR will have extensive PvE parts. And I wonder why that has to be that way. Why not make a pure PvP game, without levels and gear progression, where the classes are perfectly balanced for PvP without having to take PvE into account? Why not make pure PvE games, like Everquest was? Why not make a game that is all about raids, where there are no levels, and nobody is forced to solo 70 levels just so that he can join his friends in a raid? And why not make a game where you only solo until the level cap, without tacking a raid game onto the end?
I'm not saying that any of these mono-games, focused on only one of the aspects of MMORPGs, would sell as well as World of Warcraft. But it would certainly be easier to make a good mono-game than making a game with many parts, all of which have to be good. Mono-games would be cheaper to make, and even with a smaller subscription base could be perfectly profitable and viable.