Welding techniques and joinery got better. So did tubing.

Did the same books from the 80s also pooh-pooh butt brazed joints, i.e. fillet brazed "butt" joints? Didn't think so...

Plus it might be that the books you are reading weren't based on the most awesome science and technical knowledge... They might have just been bringing their preconceived notions of aesthetics into play instead of sound technological know-how.