I think it's a mix of factors. Mainly:
-The BBB factor / perceived potential scarcity
-Potential high Return On Investment
-Growth of the market for DMD pins - people who grew up with the 90s games now have our big-kid jobs and can afford them in the first place
-Visibility of Pinball; the new Sterns are leading to pins showing up in places they wouldn't have 10 years ago (but would have 20 years ago.) Call it a new pin renaissance (despite Stern's poor quality.) in part due to the fact that locations can have these games brand new.
-Improvements in the economy