I can remember bike parts catalogs from major vendors still listing Clement Campionato del Mundo tubulars - 32 mm, thank you! - as their very best tires. There were people who toured on those, even - obviously with more disposable cash than I had, then or now. Then, too, bikes sold in the U.S. that did NOT feature tubulars came stock with 27-in clinchers, which was pretty much the Anglophone world standard for high performance but not all out racing bikes. My 1976 Puch Royal X, which was built by the same people on the same assembly lines as the very deluxe Austro-Daimlers, came stock with 27-in clinchers + room for Bluemels mudguards, and I ran it that way for a while - then I got a tubular wheelset and ran those. The larger amount of air around my tire did not change anything in reality.

My opinion and nothing more - fashion, for sure, drove the tighter clearances, as did a shift in how bikes were marketed. Greater affluence was especially a consideration. Why sell a cycling enthusiast one bike that can be raced one weekend, then loaded up for a short tour the next, when you can sell them TWO separate and distinct bikes that split those functions and are ever more specialized? The rise of the mountain bike surely didn't hurt the new paradigm of different bikes for different purposes for cycling enthusiasts.

The other aspect of this is the rise (for a while there) of dedicated touring bikes that were truthfully more designed for expeditions than for shorter tours. They got heavier and built with more robust tubing and ever wider gear ranges and larger racks that encouraged carrying the world with one. Then cycletouring fell out of fashion and retailers were stuck selling bikes less suited for general purpose, multiple kinds of riding. The general purpose riders wound up with mountain bikes or hybrids, the speedier folks went with bikes more suitable for "competitive" riding, and the great general purpose pavement bike withered away.