(1)-Because we the people no longer have any say in our governance, which means that desperate public need is no longer factored into political decision-making (for absolute proof look at New Orleans);
(2)-Because such construction requires massive redistribution of wealth -- specifically fair taxation of Big Business generally -- which Big Business will never allow;
(3)-Because construction of mass transit would reduce the profits of Big Oil, which Big Oil will never allow;
(4)-Because even if such funding could be forcibly extracted from Big Business (whether by taxes or outright nationalization), the cost of land (an accurate measurement of the increasing worthlessness of the dollar) has risen to such levels that the necessary land-acquisition expenses make mass transport impossible;
(5)-Because the outsourcing of raw materials and technology has similarly inflated these costs, again placing them forever beyond reach.
The only rational mode of public transport is powered by electricity and runs on rails. But the United States -- which already has the worst public transport in the industrial world (that is, the slowest, most expensive, most viciously discriminatory, least efficient) -- is condemned forever to its herky-jerky bus system: a hallmark of the Third World, and proof positive of the extent to which the U.S. is already a Third World country. This is because -- apart from all the other factors -- building a rail transport system is impossibly expensive. Such systems cost nearly $70 million per mile in 2000:
http://www.publicpurpose.com/ut-lrt2001.htm and because of the skyrocketing costs of land acquisition, the per-mile cost has risen as much as 25 percent per year since then. Seattle's system -- which is already nine years behind schedule due to political and bureaucratic sabotage (the ruling class regards rail transport as "Manhattanization" and fiercely opposes it) -- is already costing $180 million per mile:
http://en.wikipedia.org/wiki/Light_rail(scroll down) and for that reason will never be completed.
The time to build transport systems was during the 1960s and 1970s, when the federal Urban Mass Transit Administration (UMTA) had matching-fund programs, with the federal share as high as 90 percent. Such programs will never again be allowed; American big business -- that is, global capitalism -- has no intention of ever again investing even a penny in anything that will benefit American workers. This is the bottom line, and this is why there will not be -- now or ever -- any meaningful expansion of public transport in the U.S.
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Edit: typos