I'm going with the idea that they're waiting for the Super Bowl hype to settle down, then use the gathered media for that event to start publicizing the May 4 fight. It's win-win for them. There's all this press and media already put together by the NFL, you get some reporters to ask some NFL players who they think will win, etc. All the logistics are already in place and they don't need to spend a dime.
If they announced it earlier, it would have gotten lost in the hype of the NFL playoffs. People are talking Flacco, Lewis, Kaepernick, Brady, etc. right now.