They are relatively unknown. They were not widely marketed. Even today, a Vivitar K mount body is available brand new....few people even know about it. There was also never a significant cost advantage between the Vivitar K mount bodies and the Pentax K-1000. There were also far better options - the Ricoh KR bodies come to mind...there were others.
These days, with prices on used K-1000 (and the other Pentax K series bodies) hovering around ridiculous lows...there isn't much reason to buy anything but the real thing.