It's because those of us who ride road bikes are bent at the waist. Belts or cinch cords make bending uncomfortable--or rather, they make riding bent over for long any sort of distance uncomfortable. Suspenders allow bending without discomfort. That's why bib shorts, bib knickers and bib tights are popular with roadies, but not with MTBers.

My first winter I wore regular elastic-waisted tights. Never again. I love my bib tights.