That isnt an explanation of why this is the case, merely a statement that it is the case.
Why do UKF need their (Unupgraded, by the way. A non-vickers UC beats a 221) UC to be a soft counter to the 221, despite its lower cost and quicker timing? I'm not sure of the justification behind it.
It is the case, because modders balanced it like that.
It is the case, because if you want to kill a vehicle, there already is a unit in T0 for that.
It is the case, because combat is NOT units primary role.
It is the case, because it is not meant to go against any vehicle unless it gets a jump from favorable position.
I don't know what you struggle with here so much.
Its base armor was increased, because it didn't do good enough job against its intended target-infantry and that's it.
I don't know how to tell you its not supposed to fight vehicles in a way that you would understand if you don't get it directly.