In the U.S., the mainstream move from 27" clinchers to 700c clinchers occurred in the mid to late '80's. Big improvements were made in mainstream clincher rims/tires at that time as well, with hook edged rims hitting the mainstream being the biggest advancement. True racing rims/tires had been 700c all along, in the form of tubulars.

I think a lot of it was marketing......clinchers became more high performance (high pressure tires on hook edged rims available in the mainstream), and therefore a little closer to a tubular in regard to performance. Marketing, but 700c equated to racing tires, clincher tires/rims improved, so the switch was meant to associate the improved clincher tires/rims to the 700c racing tires.