Originally Posted by
schnee
It didn't used to be that way. Early on, the steel used lost a lot of tensile strength when heated, and since brazing requires far lower temperatures than welding, the tubes and butting could be a lot thinner on lugged frames. Steel has come a long way since then though, so the differences aren't nearly as pronounced.
Umm, 4130 is still 4130. And has been since at least the 20's or 30's. You can get it lugged or welded today. For that matter, weren't the penny farthings welded?
Reynolds 531, OTOH, I understand did not lend itself well to welding.
But... Pretty sure the answer is economics. If you can source 3 welded frame sizes to fit the biggest chunk of the market, and get some differentiation by using a slightly different size tube, why go lugged? Large diameter tubes are what people expect from a road bike. Sloping top tubes too. Part of the side benefit is disposable components. If there is no standard size of anything to bolt to, derailleurs, shifters, and other components can be changed more easily year to year forcing the old stuff to be dumped. Solves lots of problems.
I'll betcha low spoke count wheels have an origin in economics too ("if we can save $0.75 cost in spokes on say 200,000 wheels a year and the quality doesn't totally tank, isn't that good enough?").