Not that this is a sure-fire explanation, but off the top of my head I can think of at least 2 industries that would have vested interests in suppressing permaculture: pesticides & fertilizers. Annecdotally, many farmers here in France are straight up taught their trade by instructional material made by fertilizer, pesticide, and seed/grain manufacturers. So they might not even be aware of potential "better" options.

An imperfect analogy is that legalizing cannabis requires less labor & more funds than keeping it illegal, yet we're still hesitating to do that in many countries.

Humans (and thus most markets) have other incentives than simply having things for cheaper.