I get why you would make the assumption, but markets are not always perfectly rational. Big brands might not want to take the risk, small brands might not gain

I get why you would make the assumption, but markets are not always perfectly rational. Big brands might not want to take the risk, small brands might not gain the traction as it's not enough of a determinant to make people buy something completely unfamiliar or associated in their minds with quality/good fit etc. (The women complaining about this are probably still buying jeans, so it's also not clear that money is actually being left on the table, they're just dissatisfied with what they are still buying.)