But it seems an obvious market failure that I can't go into a store, pick out a left cup and a right cup and the straps of my choice, and walk out with something that will work better for me than anything I could find in Target without significant extra expense.

OTOH, look at the signaling implications of such a purchase. There's a big difference between knowing you're asymmetrical, and going and buying special clothing because of it. Sure, some people will buy it, but it seems unlikely to achieve mass acceptance.