Quoted from AlexG:Going back 20 years there is only one answer, Star Wars Episode 1. There can be no debate. It was a hugely anticipated movie and a brand new pinball platform full of potential.
What we got was a lousy movie full of uninspired special effects, a symmetrical play field with an uninspired rule set and the game that was directly responsible for the death of Williams pinball. That game was the start of the lowest point for the industry and nearly killed pinball.
There are many other reasons for the death of pinball but I think it started with the acquisition of Williams by Bally along with the dumbed down rule sets put forth in the WPC95 games. They were trying to appeal to the lowest common arcade denominator and ended up turning off the hardcore pinball players.