Similar to the power of the Raleigh empire's impact on resisting conversion to 700c in the UK, which someone has just mentioned, the Schwinn dynasty in the US was deeply entrenched in pushing their own 27" rims, wheels and tires for a big hunk of the after-market pie. However, I think another problem with "changing over" was the lack of serious enthusiast-directed high quality tires available... in either size. From our mindset today we view 27" tires as cheap, lower quality, heavy beasts, whereas we can easily find expensive top quality 700c racing clinchers. But, there was not this clear a distinction decades ago. Tom Avenia in NYC (who imported Frejus bikes, Campy components, etc.) would sell me 700c tires for a bike I'd brought back from France in 1971, and usually had 700c rims available too. BUT, he always shook his head when I requested a pair of 700c tires (which I always would tell him were just for my fictitious sister's bike) because even the Michelins and Clements available in the '70s were commonly heavy, wire bead, cotton belted, gumwalls, and everyone seriously into cycling knew that sew-ups were absolutely superior, and were the standard for even amateur racers and those who considered themselves serious cyclists, in the US. This issue continued long after Mavic released their beautiful light weight "Module-E" rims (c.1975). These looked very much like Montlhery tubular rims, and had full stainless eyelets, narrow profile, double walled construction, polished surfaces, were for modern hooked bead tires. They were simultaneously released as both 700c and 27" and weighed only around 460 grams... which is somewhere between Open Pros and MA3s, today! But (and please do correct me if I'm wrong here), it was well into the 1980s before "Serious" higher quality and eventually even folding clinchers tires gradually began to appear - in either size. So, my LBS would still groan and then "special order" 700c tires for me if I didn't mind the wait, until at least as late as 1982. But, they never really stocked them - even for any of the Raleighs, Atalas, or Peugeots which they sold... all of which were still targeted for the US export market and came to them still equipped with 27" wheels.
Bob Hanson, Albuquerque, New Mexico, USA