Why is white bread so much cheaper than high-fibre bread?

Here in NZ the cheapest bread is white bread, made from refined flour. It is also the least nourishing, so now bread prices are rising rapidly because of the Australian drought, among other reasons, poor people are being forced to eat this junk food. Last time I was in Europe, high-fibre breads like rye bread were cheaper than white. The only reason they were unfashionable was snobbery; the lower cles ate them so they must be bad. Why is the situation reversed on this side of the world?