“they, They, THEY!!!!” Omg, guys, loosen up the tinfoil. No big conspiracy.
25.4 (not 26.0, unless you’re French) was the clamp size when bikes were based around the 1” threaded headsets.
Once the 31.8mm threadless became standard, bar clamps moved to 31.8 along with it. That was like 20 years ago, so it’s not like it was something that ‘just happened.’
Grip diameter is a whole other thing. MTB straight bars are typically used with a thick rubber grip, so the bar tube is thinner, to keep from making the grip too fat.
Road bars are usually taped, which is a thinner layer than MTB grips, ergo, the bar is a bigger diameter.
Besides; the form and function of most of the controls ( brakes and shifter) don’t really work if you go from flat bar to drop bar, so WTBD?