What took so long?

Apparently GM had hybrids figured out by 1969:

link

Yeah, well, there were a lot of innovative ideas around and the carmakers bought them out or otherwise took them over and sat on them. They did not want to disturb their little empire. Now they are bringing them forth, slowly. Yeah, great folks, aren't they?

*snort* Indeed.

This was conceived immediately before the energy conservation era. Just a couple years later and it might have led to something.