TBC, Vanilla servers! Why is it impossible?

I always tought about, why blizzard didn't make TBC, and Vanilla servers... and i'm not alone i think. I read a lot about it, they always wrote they can't make it, but why?
I bet there are millions of people who want to play on these servers, and i think it would be a huge money for blizzard too.

Write here all of your opinions, ideas :P