It's also because every time a good heel develops, WWE either turns them face or buries them.
Sheamus, Miz, Swagger, Del Rio are all great heel material. But, WWE continues to squander their talent. That's why we keep getting stuck with Orton and Helmsley all over again.
They really wasted Swagger in my view. Yes, I know about the DUI/weed thing (see my Swagger blues thread). But, how many times has Orton been popped but ended up World champ?
Heck, a year ago, Orton couldn't beat Swagger to save his life. Now, a Orton-Swagger match would be considered a joke.