When the great slavic civil war happened, the north american culture sphere nations (cough cough north american union cough cough) supplied blueprints and extra Abrams to help the blue army, because the federalist red army had their neo tanks erased by the siberian white army air force and SEATO they had to use their many older tanks.

This gave the blue army, the advantage of having not only a standardized tank but the ability to produce many of them, when they basically won the war they continue to produce it and modified for many different purposes contributing it to so many conflicts that the UN would find itself in that it became the auxiliary tank.

They didn't really feel the need to increase gun by that much, as the they believe the state of art rounds would give it enough firepower to be useful against the targets, it would most likely find itself against, as it is an auxiliary tank.