Actually Zakk DOES know all the scales and techniques, but for Black Label Society or Ozzy, he prefers to stick to one style of playing because he thinks technicality gets in the way of feel and emotion
I'll agree though, doing the same thing over and over again gets old, and I'd like to see Zakk do something else, but I always hear people say shit about how a band changed their style and how that pissed them off. Then I hear shit like "Oh Band A needs to change their style, it's so old and boring."
True musicians go into music for the music, not for sucking up to the fans or what people or what the record company asks of them.
That's the reason why I like Metallica and Zakk so much, they do whatever the fuck they want.