Sure McDonald's sells more hamburgers than any chain in the world -- billions -- but they don't taste as good as one made at home (nor as healthy).
Size itself doesn't make the best, size just shows people playing is what is more convenient (when was the last time you saw an EQ2 game box at Wal-Mart to buy?).
People play WoW because it's there and low maintenance for them (don't need a fancy computer to play) and it's a simple game. It serves their purpose.
How WoW retains players is that the players don't want to try and give other MMOs a chance, as they have their convenience before them. They want WoW in another game that isn't WoW, and get dissatisfied and come back.
I like the other MMOs as they are different from WoW and each has something very unique to them that's fun -- EQ2's crafting system; guild halls and housing; friendlier community; commitment to stick to things and not quit...and their world isn't destroyed; DDO's multi-class builds; their dungeons (now THOSE are dungeons, some with 6hr average run times!); and loot system; RIFT's dynamic world; true exploration achieves...and shinies even on top of mountains!
There's room for more than *just* one MMO.