when the new update dropped new abrams were added to the game featuring new armor models and volumetric, how ever the only pre-existing abrams that recieved the new model was the base M1A1.
This leaves the M1A1HC (all variants), M1A2, SEP, SEPV2, M1(&KVT), IPM1, M1A2T, and the M1A1AIM (all variants) without any of the new models or armor.
I have no clue why gaijin decided to do this. my only guess is gaijin wanted to test the new models before applying them all, but knowing gaijin this may not be the case.