Soldiers have been asking them for years. At least one National Guard Unit took one to Iraq and used it (And was reprimanded for using it, How dare that unit bring in a superior Vehicle WITHOUT checking with the Army high Command first). Now some units do have M113, mostly the Armor and M2 Bradley Mechanized Infantry Units (The M113 is used in support roles for example in the form of 120mm Mortar Carrier variation of the M113, in the form of Maintenance units for the M2 Bradly and M1 tanks). As to regular units, they have to use up armored Humvees, whose off-road performance is further restricted do to the extra weight of the Armor most are carrying now (But these Up-armored Humvees still use less fuel, tires can last up to 20,000 miles in the up-armored role, and can be hauled by the normal tow vehicle the Five ton Wrecker).
The M113 is a track vehicle so it can go into the sand (i.e. get off the roads), it has enough armor to protect soldiers from .50 caliber weapons (Which means most of the IEDs would barely damage them) and designed form day one to carry the weight of its armor.
The downside is the M113 use more fuel, its tracks only last about 2000 miles, needs daily maintenance especially greasing the tracks) AND you have to upgrade the support vehicles (i.e. Tow Trucks, the Five Ton Wrecker can be used with up-armored Humvees, but NOT with the M113, the M113 is to heavy).
The biggest reason the M113 has NOT been shipped to Iraq seems to be that the US Army does NOT think it can support the number of M113 that would be needed. Gives you an idea of how stretched the US Supply lines are to support the US Army in Iraq. The M113 was used extensively in Vietnam, and proved it s worth. It has several weaknesses, first it is NOT a Tank (I weighs just under 13 tons) has Aluminum armor (Which tends to catch on fire when hit with a RPG round, but this has been addressed by the addition of Reactive Armor and other improvements since Vietnam) and has no way to fight from inside the Vehicles (But the same can be said of the Armored Humvee) and gets about three miles to the Gallon of Diesel fuel (Compared to the Humvee's 10).
For more on the M113 see:
http://en.wikipedia.org/wiki/M113http://www.fas.org/man/dod-101/sys/land/m113.htmhttp://www.globalsecurity.org/military/systems/ground/m113.htm