My take on this is that pinball has, to one degree or another, been a counter-culture thing. There was always something a touch rebellious about it. You found machines in bars, pool halls, newstands, and soda-shops. It was always associated w/ gambling, and wasting time.
By the 70's this was blatant--rock musicians, not just just hot but suggestively posed chics, back-glasses that looked like they were straight off a rock album or comic book, or high-end sci-fi fantasy paintings (Centigrade 37, Paragon, Medusa, Embryon, Arena, Fathom, Blackout, and more). Some of this art is a direct tribute to, or rip-off of (depending on how you look at it) comic book art. By the 80's, the easiest way to appeal to consumers who were somewhat counter-culture and still be commercially viable was to follow in the foot-steps of comics and fantasy creators. That, and I know a lot of pinball guys are also comic fans.
That's my take on it. As a comic guy, I may be biased.
Just to prove a point, though. Check these out (don't know why I've never seen anyone else point these out). The comic is almost 20 years earlier--great Jim Steranko art. The backglass doesn't do it justice.
