Why don't "commanders on the ground" ask for more troops?

At the beginning of the war, Gen. Eric Shinseki privately challenged Defense Department troop estimates for occupying Iraq as too low. Defense Secretary Donald Rumsfeld was enraged and leaked the name of Shinseki's successor to the media a year before the end of his term as Army chief of staff, thereby making him a lame duck. Last May, the Baltimore Sun reported that, based on a flimsy pretext, Maj. Gen. John Riggs was stripped of one of his three stars just before retirement for more publicly insisting that the military needed more troops.

Shinseki and Riggs are exceptional only in that they spoke out publicly. Their view seems to be the consensus. A recent report in the New York Times was typical: "Commanders concerned for their careers have not thought it prudent to go further, and to say publicly what many say privately: that with recent American troop levels — 139,000 now — they have been forced to play an infernal board game, constantly shuttling combat units from one war zone to another, leaving insurgent buildups unmet in some places while they deal with more urgent problems elsewhere."

from the LA TIMES