There is a good deal of unused physical space that could be used for housing, but is currently being used for agriculture, or simply for speculation. One reason it isn't is because the tax system discourages sale of land--annual property taxes are low compared to sales and inheritance taxes. This restricts the supply of land available for development while it also makes it easier to use land as collateral for speculation--as in part happened in the late 1980's.
Among other things, I'm sure . . .
Christian