I understand the movie has a big following with those that grew up with it but it doesn't seem to have built a new audience with a younger generation. Other 80's movies like Back to the Future and Ghostbusters have done this but that's rare.
Below is a video where college students are asked if they know certain 80's movies. 4/8 knew The Princess Bride by just the title whereas 8/8 knew Back to the Future and Karate Kid. With how much a pinball machine costs to produce, especially at the level JJP does, I think there wouldn't be enough sales to make it a profitable title.
JJP would probably sell 10x as many Back to the Future trilogy pins as a pin based on Princess Bride.