Professionals aren't developing at home unless they have a darkroom, which were fully capable of doing color negatives in the 1970s. If you can shove color into a self-developing mobile consumer device, a professional can have it done.

Somehow this needs to be about poor constrained do-it-yourself professionals?

   You can process and print B&W yourself. Color needs to go to a lab.

The consumer "street" photography of the time was mostly Polaroids, or little tiny box cameras you had developed at a booth.

This has all to do with a feel, and It could be that it was mostly shot in color and converted to B&W. Or, the wonderful dynamic range of B&W could be the reason. That's fine, but it has nothing to do with lack of choice.