I think it started with IBM: System/360 and /370, PS/2, OS/2, PL/I

And then Gary Kildall also seemed to like it with CP/M and PL/M, but those were after IBM had used it and I'd guess Gary was just copying IBM.

Between just those two influences you cover a huge portion of the mainframe and micro computer worlds during the 60s-80s

It was a convention to denote a variation or version. Not sure how the trend started though.

Maybe referencing the reputation of IBM System/360?