Better tubing, better fabrication techniques, better welders?
And this may well be the case.
I just think far too many people think building the lightest bike possible is the way to instant success.
That chro-mo frames and alloy fenders are what differentiates a 1st place bike from an also ran.
The top factory bikes in "production" racing are always going to be built regardless of expense or with the object of using the bike for many years. I suggest this is why KR's had "special" frames and did not use production K frames.
You have to give the buyers of the "race" bikes something extra for their money or they might as well race a stock bike.
Also, hardly makes any difference when the rider weighs in over 200 lbs.