Here's my theory about thebigspendur's theory.
I don't question anything he's said here. But there's one thing missing in this history. The invention of the safety razor, and its swift ascendancy during World War I, when a whole generation of men used them in the trenches. The whole thing about the DE was that a man could shave himself with unprecedented ease. But the barbershop shave didn't go away for a couple more generations at least. Straight razors made in the 20th century have mostly been destined for barbers' use. These guys are professionals. As many a newbie on this site has noted, a nice heavy wedge is a lot easier to use successfully when you're an unskilled beginner. As many a pro (like mparker and the bigspendur) has noted, on the other hand, once you really know what you're doing you can get just as good a shave from a 5/8 extra hollow as from a 7/8 wedge – and the hollow one's more maneuverable and arguably more precise. A good deal of skill is key, though, as is a very sharp, expertly honed hollow razor. Wedges are more forgiving on both counts.
So there you go: wedges were still popular when men were still, in significant numbers, giving themselves straight-razor shaves. Remember, these guys weren't hobbyists like us, spending half their waking hours pondering the art of the shave. We're nostalgic for the days when fathers taughts sons how to shave, but in all likelihood you can learn much more from Randy, Lynn, Joe, and others on this site than guys back then could learn from their dads. Prep, technique, razor maintenance etc. were probably pretty hit and miss, most honing done by someone else; and in terms of overall skill probably most guys then just weren't as good at straight-shaving as mparker and bigspendur are now. So the wedge grind endured through the 19th century.
Once the safety razor took over the market for self-shaving, and straight razors became mostly professional instruments, it was perfectly natural that smaller hollow grinds would come to dominate. They're much easier to hone (and if you're shaving 30 faces a day you have to be able to do your own honing); they're easier to manuever precisely, and they give a lot of feedback.
Just a theory.