There are eight different, incompatible 26 inch wheel sizes.
1) 26 x 1.25: bead seat diameter 599mm. Obsolete.
2) 26 x 1 1/4: bead seat diameter 597mm. Schwinn S6. Obsolete. Still made for very old Schwinn lightweights, but uncommon.
3) 26 x 1 3/8: bead seat diameter 590mm. Traditional British three-speed size. Also known as 650A. Obsolete for new bicycles in the USA, but very widely available in bike shops, hardware stores and big boxes in America you can still buy these tires in far more places than the industry standard 700Cs (bead seat diameter 622mm)! Common in Japan where it is used on new bicycles, and common legacy size in the UK. This is also a common wheelchair tire size.
4) 26 x ?: bead seat diameter 587mm. Very obscure and obsolete proprietary size.
5) 26 x 1 1/2: bead seat diameter 584mm. Also known as 650B. Obsolete, but a few companies are trying to revive this size for both road and mountain bikes with proprietary tires they provide.
6) 26 x 1 3/4: bead seat diameter 571mm. Also known as 650C. Obsolete. In the USA these wide tires were used on old Schwinn middleweights. Tires are still made, but uncommon.
7) 26 x 1: bead seat diameter 571mm. Known as 650C as well, this narrow racing format is based on an obsolete Italian sew-up tire rim size. Used today on time trial and triathlon bikes.
26 x 1.0 to 2.35: bead seat diameter 559mm. The ubiquitous mountain bike tire size, descended from the American balloon tires of over a half century ago. Now made in a huge array of widths and tread patterns. Widely available worldwide in widths of ~1 ¾ inches and greater; the narrower sizes are available in well-stocked bike shops. In the wheelchair world, this smallest 26-inch size is sometimes referred to as 25-inch.
Based on the combination of strength, weight, availability of tires and rims and flexibility of frame geometry, Im befuddled as to why modern loaded touring bikes dont use the 559 as a standard size.
Best,
tcs