The reason the T-72s were so prolific in foreign nation during the late year of the Soviet Union and after it's fall was due to the T-72 having models specifically for export. Notably the T-72M and T-72M2. As to why the T-64 was never treated in this manner, the T-64 was a far more complex vehicle and was actually designed alongside the T-72. The T-64 was a better tank, the T-72 was a cheaper tank.