The Pro Bowl participants are decided by a vote by the fans, players, and coaches (one-third for each). And the fans have the right/ability to stuff the ballot box, as there are no limits to # of votes. Being that Tampa Bay was such a horrible team this season, not many even had the opportunity to get to know David to vote for him.
Also note, 20+ players will not play in the Pro Bowl, due to playing in the Super Bowl the following weekend, or the many who simply opt out because of injury (or in some cases, "injury"). When this happens, David will be asked to serve as a replacement and will, indeed, play in this season's Pro Bowl.