rare doesn't necessarily mean good, in fact most of the time rare even means crap. 
Well, Ully, same old story and not quite correct. Rare potentially means a lot of things, for example:
- shelved when in prototype phase
- no success in market due to low PR power (small companies' curse)
- no success in market due to economic crash
- rare because of flaky hardware causing too much down time for operators
- being freaking old, therefore market wasnt big and low numbers made
- being just bad
- being just too good
...
These two games have amazing artwork and are "lost" games of small (small as in not Atari/Kee) but pioneering companies that released awesome (good) games overall.
