I guess I am a tech elder now, and the reason for gold is that it was corrosion-proof and is easier to solder and wire-bond (the gold was present in more than just the cap). The older process (prior to gold) with tin and whatever else, was prone to oxidizing and corroding.

Gold was the best option at the time vis-à-vis the drawbacks of the others and technological capabilities. Not needed any longer as the tech matured. I know this stuff because I read about it on some website back in like 1997 or so (maybe HardOCP).