0.100 was ubiquitous in through-hole and using a .100 edge connector also meant you could use a socket that connected to the mainboard with pins at a .100 spacing - that's what everyone had experience using, and it probably also was easiest to develop PCBs with .100 parts using the prototyping and design tools available then.

Mostly is because of mechanical tolerances. Fine pitch on edge pins is highly prone to shorting out or even being misaligned by an entire pin space due to tolerances required for insertion, and in some cases even thermal expansion could come into play.