The land isn't being used because there is no pressing need in the market for more land use. I don't think people realize how much agriculture yields have improved since the 1950s. It is astounding really how much yield we get per acre now mostly from leveraging hybrid species actually, that has been a far bigger improvement to yields than even fertilizer use which is probably what most lay people think is the source of modern yields. In some crops and regions its on the order of 10 fold improvement in yield per acre.