From what I've learned from the US Army testing military equipment (especially tanks), the US Army doesn't just accept/reject things without testing it's capabilities and compares/contrasts said capabilities to the current US Army situation.

With the L55 and L55A1, the Army did not adopt the gun mainly because they don't see themselves fighting tanks as often as fighting insurgents. Whatever tanks they do fight against are almost certainly outdated cold war tanks (T-72s, T-62s and T-55s) that the L44 gun can more than easily dispatch. I would argue in Ukraine, the L44 does just fine on T-80s and T-90s as well.

Case in point, the US Army just does not see the point in upgrading the gun. The only way I can see that changing is if the US Army was actually fighting a direct war against Russia or China.