If you've ever delved into bike frame design a bit, you've probably realized how finely calibrated everything is, and how changing the size of wheels would throw all kinds of things out of whack in a hurry. My guess is that the Continental constructeurs and English ones arrived pretty much at the two very close wheel/rim sizes because it made frame design efficient and simpler, to accommodate the widest variety of sizes necessary with the fewest modifications to the frame configuration.