observation over time>>
# accounted for on pinside, pinball database, talking with other rare/super rare collectors over time and that have been in the hobby for decades, talking with operators over time, understanding that it was not popular in US and most went overseas, understanding that most GTB of this era did not survive overseas or make it back alive in containers, understanding that these likely went into some rougher locations based on theme alone.
It really comes down to unpopular games from the 70s and 80s that had a larger % go over seas, means even fewer alive today. US collectors drive the collector market and when they did not even know of the game then they did not bring them back.
Keep in mind that some older collectors have also said this was a time when companies were lying about produciton numbers to make things seem better than they really were. They wanted operators to think there would be parts and the company was stable.
Gameplan supposedly did this also in the early 80s when things were rough before the end.