I think all of the above.
I also think the ease of finding machines for sale due to the internet, ebay, clist etc... has helped too. 7 years ago my buddy gave me a non-working Meteor he found in his basement. While trying to get it fixed, I found out just how easy it was to buy a pinball machine. Up until then I thought you had to know someone who knew someone who owned a bar or route and didn't want one.
I would also add that MAYBE the increase of Stern producing titles with a high fun factor is causing collectors to buy more NIB pins.

Good point. And not just anti social but less physical. I had a buddy's kid turn down a quick job because he "didn't like to sweat."