Originally Posted by
Moonlapse
There are a lot of reasons. One is balance. I wasn't around for WotLK, but I've heard the stories about how long it took to get DKs in line.
Another reason is what archetypes are needed in the game. Currently we need more tanks, which was the goal with making Death Knights. Well... almost 3 years later and we still have a tank shortage. Releasing another tank class would be kind of weird, and releasing another DPS class would only hurt the problem. They could release another healer, but what we have at present currently feels really good (ie: One healer of each armor-type).
There are lots of ways to improve the game without adding new classes. I'd be more in favor of offering additional character customization than adding new classes.