Because there are different options available. People can argue which game is better but what matters is the time people have invested into each game. Lets take wow as a prime example, lets say you joined wow in Wrath, from then 2008-2013 that is 5 years of time invested into the wow culture, character and lore. To just say drop all of that and join another game (in this case Rift) isn't worth it to a lot of people and that is why wow remains #1 in my eyes. It has nothing to do with which game is better it is to do with either continuing years of hard work and achievement to drop it and join, what could possibly be a game that is worse.
That's just my view, I tried Rift and quit because the story did not engage me, if a story cannot grab me I don't play no matter how innovative it is, look at Guild Wars 2, awesome mechanics, events etc but the story is just boring for me.
---------- Post added 2013-04-04 at 03:49 PM ----------
Yes it is but wow now compared to 2004 is a lot different. Being a clone is fine as long as you can evolve from your foundation. Like some of the clones in Star Wars. Some of them defected, went there own path and that is what wow has done and continues to do.