Probably because they think a World Series title with Cliff Lee in 2010 is better than the chance that Montero will be a great hitter, which everyone says he could be. It's a tough call.
Ken Davidoff, on his blog, gives the insta-opinion that the Yankees should wait and sign Lee as a free agent.
The Yankees sure do have a lot of catching prospects, starting with Francisco Cervelli in the majors. They have Austin Romine -- or August Romaine, as Mike Francesa likes to call him -- in Double-A.
The Yankees also have Gary Sanchez, who is hitting .385 in the rookie Gulf Coast League. Obviously he's a ways away.
Gotta figure the Yankees are comfortable with Cervelli/Posada for 2011, and then Romine/Cervelli after Posada's contract expires.