It's such an apples-vs-oranges comparison, and it's also largely a personal choice. I do play the card game over the online game, though, regularly.

People who've played lots of Magic say that the WoW card game is a better game than Magic, for what it's worth.

EDIT: Oh, I misread your question. Why? Well, it's sort of like the difference between driving a car and riding a bike. A bike isn't a crappy car, it's fundamentally different, with its own set of strengths.

Also, the collectible card game is NOT a simulation or re-creation of the online game. It's more like Magic set in the WoW universe.