I would say that B corps is a global movement. There are 1,220 B corps, varying from small shops of one or two people to large multinational corporations. One of the largest B corps in the world is the Business Development Bank of Canada, so it's a really broad spread movement.
If you asked those individual entrepreneurs, you'd probably get a better answer than I would be able to give you.
More generally than not it's more of an economies of scale sort of thing. The vast majority of B corps are just not positioned to penetrate those markets because they're not large enough, but the upper tier probably would.