Because fat people can't wear them.
Yoga pants sell a fantasy, you're young, athletic and sexy, and anyone can buy into that fantasy by buying a pair, and even better anyone can wear them. Yoga pants don't care if you're 400 lbs, they'll stretch to cover that XL ham you call thighs.
Breeches though? They sell a similar fantasy, your young, sexy, in charge, but breeches...breeches make you earn them. Breeches aren't covering that Supreme pizza you ate last night. Breeches are gonna laugh at you, dare you to get in shape enough to wear them.
So the fashion industry pushes the thing it can sell to everyone.