[WS:] You need to look at it from a broader perspective. As David Harvey argued, the Left pretty much fell into the neo-liberal mode of thinking that emphasizes individual freedom. This resulted in primacy of individual accomplishment and self-actualization at the expense of social solidarity. In such a mind frame it is easy to perceive heroic individuals as prime movers of progress - perhaps not on such a delusional scale as in Rand's Atlas Shrugged, but still emphasizing the individual over the team of which he or she is a part. So in that culture, it was quite easy to see Obama as a "savior" especially after the eight years of nightmarish Bush administration.
So the unrealistic expectations that you mention are really a product of the neo-liberal Zeitgeist in which we live and which, as Harvey argued, dominated the mind frame of many post 1960s lefties.