Something about increased compression forces. I read it somewhere but I can't find it. Wheelbuilders put in more tension on a clincher rim, all other things being equal. I think.
Eta: Cav also broke spokes the day before yesterday.
Tubs constrict onto the rim and reduce spoke tension a bit but that's not really a good thing.
Up to the 1990s, it was fairly common to find 36 spoke wheels on top race bikes, often plain gauge. It wasn't for strength - it was so that a wheel with a broken spoke didn't have to be changed if a spoke broke, because a plain gauge 36h wheel stays true enough to be ridden with only 35 spokes. Spokes are vastly better now.