The big thing is that it was one of the last WPC machines released. IIRC only Cactus Canyon came afterwards, and many people feel CC wasn't 'Finished'.
So MB got the mystique of being the LAST of the finished WPC machines on the market.
Add in how, at the time of release, pinball was on a big downswing. So MB's sat longer at distributors, spent less time on route, and generally hit the collectors market in markedly better condition. Yet production numbers weren't very high. Compared to games like T2, Funhouse, TAF, TZ and such we're looking at a BIG drop in available machines.