Ohh thats quite easy to answer:
Because germans are not supposed to have any advantage over soviets late game and panther had ALL the advantages, speed, cost, AT, armor, health of heavy tank for only slightly more then KV-1 price.
The thing about the kv1 is that its very good against infantry while the panther is only useful against tanks. dedicated unit must always be more efficient then the generalist ones otherwise their is simply no reason to buy one.