There are a lot of factors that made World of Warcraft so popular, but I don’t think you can discount the fact that the game is just really easy to run on basically any computer for a long time. When a lot of games required decent gaming PCs but WoW would run on a TI-83 glued to a piece of cardboard with “COMPUTER STUFF” written on it, there was a natural advantage there. And at this point, well… computers have advanced. Lots of cheaper low-heft PCs can run MMOs easily enough.
For some people that’s enough. Sure, everything is on low settings and your frame rate could generously be described as “movie-like,” but the game runs and that’s what matters most. Other people, though, want their games to run as well as possible. If there is an “Ultra” setting it should be turned on, and if the game ever dips below 120 FPS you need a new computer no matter what number of frames the human eye can actually see. So how important is your computer’s performance in MMOs to you? Do you demand the highest fidelity possible, or are you happy with a rig that runs decently?