Why ? They have been the bulk of UA tank force and proved to be quite good.
Additionally, T-64 were high-end, not for export models, unlike several years younger T-72s which are the bulk of your rus nazi tanks.
The only reason why the rus nazis decided to focous on the T-72 platform was that a factory producing T-64/T-80 was in Ukraine.
It introduced a number of advanced features including composite armour, a compact engine and transmission, and a smoothbore 125-mm gun equipped with an autoloader to allow the crew to be reduced to three so the tank could be smaller and lighter. In spite of being armed and armoured like a heavy tank, the T-64 weighed only 38 tonnes (42 short tons; 37 long tons).