M1101/02 trailers are not being phased out. We just have too many. As we deactivate BCTs and deactivate functional brigades we only move some of the personnel authorizations to other BCTs and overall the number of personnel goes down. Personnel are tied to vehicles, and vehicles are tied to personnel. If a unit loses personnel authorizations they lose the vehicles required to move them. If they lose vehicles they also lose some personnel required to sustain the vehicles, then they might lose a couple more vehicles required to transport those sustainers. Formulas are used when TOEs are created to ensure the correct MOSs are authorized and the correct number and type of vehicles are authorized to meet the units mission and achieve the desired mobility factor. Units then turn in or requisition equipment to meet the authorizations in the MTOE. Or something like that.
We also created a massive property nightmare for ourselves over the last 12 years first from the BCT transformation with all the reflagging and moving, then with two simultaneous wars and constant combat rotations. Our buildup into two theaters, rapid expansion of the Army, near constant modernization initiatives, then subsequent closeout of two wars further complicated things. Equipment is worn out, a lot of units have more stuff than they're supposed to have, and we just don't need as much stuff as we did a few years ago. Thats why you're seeing so much get surplussed.
Oh yea, everywhere I've seen an M101 since about 2006 it was in lieu of a M1101/02. M101s are no longer required equipment, but they're hanging on filling M1101/02 authorizations because staffs, commanders or loggie types haven't had an opportunity or just haven't done the work to get them replaced. Or they're FOI and aren't accounted for properly.