What do you mean by "the short window when the labor to do so was affordable"? Other cities in the world seem to be able to build underground railways just fine and they have similar labor costs as the US. See Paris or Sydney for cities that have created new underground railways recently.
But my comment was a bit tongue in cheek - it is mostly political dysfunction. Of course the US could find people willing to work for less than $400/hr or whatever, but there is an incentive disalignment.
