I've heard a combination of things. Addams Family was a huge hit and stayed on route for a long time.
Diamond Plate made it so playfields didn't wear out with decent maintenance -- so the great 90's games meant ops didn't have to buy new pins. Killed by their own great product.

And the European market started shifting to arcade size gambling-type machines. I can't remember what they were called, but they became very popular and forced pinball out of a lot of locations in Europe.

that, plus the other factors listed above. Perfect storm. Glad it's on the upswing now, though.