Since the BTR-series of vehicles were based upon the chassis of the Soviet ZIL-151, and later the ZIL-157, and since they already make well-regarded kits of the ZIL-157 truck and its Chinese counterpart, why don't they go ahead and come out with later BTR vehicles, since they already did the engineering work for the truck chassis already? I assume there would be a big enough market for that particular series of vehicles, their being used in Vietnam, the Middle East, etc.

Why do they not release the almost-finished-two-years-ago T62 and BMP-3 kits?

Well, those vehicles require completely new research and engineering, while with the BTR, much of the scutwork is already done.