The Rock has proved himself to be a huge Hollywood star over the years. So it is no surprise that he simply does not wrestle anymore. Apparently, he had some dispute with Shawn Michaels backstage which he never got over.
On Wrestling Observer Radio, Dave Meltzer revealed why the Rock always refused to work with Shawn Michaels and how Shawn might have had it coming. Once when the Midnight Rockers were in Hawaii, there were problems in the dressing room between Rock and Shawn.
Later when The Rock started getting big, there were some people who did not want the Rock to become a star. This included Shawn, and it was pretty much to protect Triple H. So it got to the point where when Rock did get big, he did not want to work with HBK.
Meltzer also said that when the first match between Steve Austin and The Rock happened at Wrestlemania, Shawn was trying to get The Rock replaced by Mick Foley in the match, in order to keep him down.