I guess it was to get around the rear wheel hitting the straight seat tube on steel frames with the steeper angle used on Tri bikes without using really long stays before they started using curved tubes ? 650 was used until much more recently on smaller frames
Maybe it was just a trend back then. They clearly offered no overall benefit over 700c otherwise we'd still be riding them along with 26in MTB's as well. Gbj_tester will surely have a proper answer
I guess it was to get around the rear wheel hitting the straight seat tube on steel frames with the steeper angle used on Tri bikes without using really long stays before they started using curved tubes ? 650 was used until much more recently on smaller frames