There is no way parallelism was the reason for fork. It was a PDP-7, so there was no parallelism in the first place. And even worse, I believe only one user-mode process was in memory at a time; any process switch meant swapping out the current process and swapping in a new one.

There was also no form of interprocess communication other than the filesystem, nor any synchronization other than waiting for a specific process to exit. So having multiple processes work on the same problem would mostly have been very awkward.