WoW has 6 years of post-release development, which includes: optimizations, improvements, and additions. If every other MMORPG is similar enough to WoW, why stop playing a game you've invested thousands of hours into to play the same game you are leaving?
- Not everyone has invested thousands of hours into WoW
- Some people might be looking for something different than more of the same in WoW after playing it for thousands of hours
- Playing an MMORPG just after release means that the theorycraft and min/maxing hasn't really been done yet so it's a little bit more casual environment
- Rift is better than WoW in some ways.