Sales tanked because after 1993 because you had The Twilight Zone (actually kind of ho-hum with players when it first released but it sold well), The Addams Family (1992), Star Trek: The Next Generation, Creature from the Black Lagoon (1992), The Getaway: High Speed II (1992), Indiana Jones, and Jurassic Park all selling really well and still making a lot on location. This became a problem because operators were going to be very, very, reluctant to pull one of those games off their floor to replace it with another and unproven machine.
Ultimately, the sheer number of great games that 1990-1993 saw killed sales for the next few years because there was no need to turn the machines over. Pinball 2000 was meant to revolutionize the product and make operators ready to clear their floor and make way for new machines.