Great Ideas(TM) emerge and people get enthusiastic about them, but fail to realize that the cost makes them difficult to carry out as intended. The Great Idea is then either watered down (Skylab, Space Shuttle) or comes to an abrupt halt due to lack of funding or for political reasons (Space Station Freedom, Constellation).

I'm getting more and more convinced that it will take autocratic nations looking for prestige, or commercial companies offering cheap and exotic tourist flights for humans to get back to the moon. The US doesn't have the political strength or stability to plan such a mission at the moment, and keep it going for the ~10ish years it takes.