It was more a matter of timing as well as the design directions of both games. Rift came about after cata launched. Cata didn't do well post launch after people found out that the steady and what some called reasonable progression was gone in favor of making everything "harder". Rift's major mistake was waiting until after the game launched to start balancing classes. There were buffs, nerfs, an era where pyromancers were the kings of PvP and so on. The nerfs alone were enough to drive some away, and PvP imbalances most likely fit the other half of that equation.
Rift had a good start, but their premise was a risky one. They set out to steal WoW players, and for a time, they did. But getting WoW players is one thing, keeping them is another. It's foolish to think you can steal WoW players and not consider Blizzard would try to steal them back.
As for MoP, I like it and so does anyone who has been around since vanilla WoW making threads asking blizzard to implement them. Still, the expansion so far has been ill received and its not even out yet. Pandaren and the re-revamped talent systems are to blame.