What I've always heard is that Disney stopped it when they purchased the Star Wars brand because the characters were now owned by Disney and they wanted to treat their characters seriously. Which to me at least, makes a lot of sense. Darth Vader is a pretty scary, tragic villain. It takes a bit away from his character to have him break dancing. Just as Disney does not allow the princesses or any other Disney character to break character, they now do not allow the Star Wars people to break character. So Darth Vader will be intimidating and threatening, no more dancing.
I know the stormtroopers do silly pre-shows, but I believe that is in character for them to be somewhat bumbling.