Exactly. Plus the common use of mastering at the time was to optimize the recorded audio for printing on a vinyl disc, and if the grooves were too deep or the transitions to Sharp it could cause the needle to skip out of the track.
If your average listener is going to be listening on a mono device then a smart thing to do would be to pan one thing consistently to one side and the other to the other as the mono needle isn't going to care where it's getting its vibrations from. That would give you more resolution and more depth for the cut, as long as the final disc was only played in mono.
I'm not saying that's the case for every recording but I'm pretty sure it has happened quite a few times back then while they were still figuring everything out.