My guess - and it's a guess - is that the swings back and forth were due to basic R&D. Various companies made brakes, and each tried to make them better than the other guy's.

The good old fashioned Mafac Racer was pretty much state-of-the-art by the end of the 1950's. It stopped better than other brakes. It also did not hurt that, because it was French, it was on the French bikes ridden by Frenchmen Louison Bobet and Jacques Anquetil, who won 8 Tours de France between them in the mid-50's to mid-60's.

By the end of the 60's, Campy had figured out how to make sidepulls that were lighter and worked roughly as well as Mafacs. They also had a finish that Mafac, Universal, and the others, could not touch. (Of course, you paid for it, too.) Mafac countered with the Pro, which was essentially a shaved-down version of the Racer. It survioved in the pro peloton until at least 1977, when Bernard Thevenet won his second Tour using the latest and greatest version of them.

There is one other matter - the quick-release mechaism. The Mafac "quick release" was to unhook one end of the yolk cable. What happens if you forget to undo the quick release with a Mafac? It ain't pretty. Until recent iterations of Campy brakes (which require a brake kever with a built-in quick release), quality sidepulls had some sort of cam device on the calipers themselves. If you forgot to reset it, it just meant that you pulled your levers further to make the brakes work. Much safer. Weinmann and others got around this by having the quick release mechanism built into the cable housing stop above the brake, but that always looked kind of . . . . funky.

Then someone figured out the whole "double pivot" sidepull design, with increased stopping power, sidepull looks, and only a slight weight penalty over singel-pivot sidepulls.

All of this strikes me as the kind of incremental change that is pretty standard in most products. Somebody builds a mouse trap, the next guy looks to build a better one. "Better" can be matter of looks, weight, function, convenience, or combination of them. In the not-too-distant future, someone will come up with the next "latest and greatest" for brakes. Who knows, it might even be centerpulls.