I suspect that the modern synthetic seal is the culmination of manufacturers attempts to move away from leather seals, which they wanted to do to simplify and speed up production. Webley tried piston rings and later the solid PTFE washer, BSA and others tried O-rings, and eventually the modern and almost ubiquitous "parachute" seal evolved.

Leather seals work extremely well, but require softening, sizing and lubricating to work. A synthetic seal, popped out of a mould, goes straight onto the piston and straight into the gun. From a manufacturing aspect it's no contest.