And the main cause for the scarcity and increase in price for vintage Pentax glass...DRUM ROLL...
The various club and individual lens threads at Pentax Forums!!!
Case in point might be the Vivitar 28mm variants. Not only has that brand/focal length risen from relative obscurity, but certain versions have generated a cult following based on reviews and photos posted here! It is not unusual for a some to generate prices at auction 2x-3x the price new back in the day.
Good glass + exposure => higher prices + scarcity
Steve