It's because he doesn't do anything long-term for WWE right now and because they have pretty much done every match they could. While Michaels against Burke, MVP, Punk, Mysterio (whom haven't had a real match, not counting the memorial match) would be moderately appealing, it doesn't do much for the company unless Shawn was dead serious about putting one of those guys over big time (not that Rey needs it, that match woud be "dream" match purposes only) and we know Shawn's track record with these things.
Shawn has every reason to leave the business. He can't give it anything else.