You can debate the merits of Romo getting into the all-star game over Manning and Wilson. Remember the votes were tallied before the three-interception finale against Washington, so that game did not play a factor in the vote.
Romo had more passing yards (4,903) and touchdown passes (28) than Wilson (3,118, 26) and Manning (3,948, 26), but he also had 19 interceptions, which tied Drew Brees, who is replacing Robert Griffin III, for the most in the NFL. Wilson led the Seahawks to the divisional round of the playoffs, and Manning's Giants finished 9-7, one game better than the Cowboys.
Even if Romo would get the nod if Manning, Wilson or Ryan can't play, there's doubt over whether he could play. He cracked a rib against Washington that left him in great pain after the game, and three-plus weeks of rest might not have been long enough to help him anyway.