I think its because they were the highest evolution of EMs. Gameplay is excellent on almost all Sys1s. The graphics (pf and bg) are simple and comic book like with a very simple pallette of colours.
The flippers feel powerfull.
Certainly reliability (before mods) is woeful, but almost bullet proof after modding.
Finally its nostalgia. These were the machines that were in the arcades when I was an apprentice. I would have owned Gottlieb if pumping 20s in counted for anything!