Wait, Why hasn't Mark Prior been called up?

Last year, just a couple weeks after a Yahoo profile on former Cubs pitcher Mark Prior's success in independent ball, the Rangers gobbled him up to a minor league contract for the rest of the year. He pitched in just one game in AAA, but the Yankees made a priority (get it?) to sign him in the offseason to start the season in the minors. Prior has pitched for several different minor league levels for the Yankees this season, and his numbers look something like this (actually, exactly like this): in 11 games and 6 starts, he has racked up a 2.25 ERA with 15 Ks (against 4 walks) with a WHIP under 1. Prior must not be healthy, because the Yankees need the pitching help.