A combination of all the reasons above, including mainly materials and techniques.

The issue with welding in the past is that, with the steels in use, a weld created a weakened area just beyond the joint, making the joint radically stronger than it's surrounding area and therefore prone to cracking there. The welds held up fine, and the tube would crack adjacent.

We still see that from time to time, but not all that often because the current materials and techniques manage it much better. It should be noted that during the same period that bike buyers were warned against welded bikes, motorcycles, airframes, auto roll bars, and all sorts of critical strictures were being welded very successfully.

There's also a bit of confusion in that many of the "welded" bikes of the period in question, weren't in fact welded, but had brazed butt joints without benefit of filets or lugs. These so-called frames were very common at the low end of the spectrum, and while usually OK, prone to failure.

Current mat