The reason is that it’s necessarily difficult to win election if you hold unpopular views. Our political system selects for candidates who have appealing messages and platforms and creates incentives for them to further adapt those messages to the preferences of the electorate. When candidates don’t adapt—e.g., Jon Huntsman in the most recent Republican presidential primary—they lose. (Which is not to say that they are consciously lying: politicians also face strong incentives to sincerely adopt the views that are necessary to receive a major party nomination and/or win a general election.)
In this case, Obama and Romney each adopted an initial set of positions that were helpful in seeking office given the constituencies to whom they were appealing—primary voters in a liberal state senate district (Obama) or general election voters in a Democratic state (Romney). As they ran for higher office, they changed and adapted those positions to better match the preferences of primary and general election voters at the state and national levels. The mismatch between Obama’s state senate district and the national electorate was quite severe, but he was protected by the fact that his position changes were mostly made under relatively little scrutiny during his freakishly easy campaign for the US Senate in 2004 and were considered old news by 2008. By contrast, Romney has been forced to revise his initially moderate positions under the hot lights of the 2008 and 2012 presidential primary campaigns, which caused him to develop a reputation as a flip-flopper with no “core.”
The difference in their circumstances—and Romney’s lack of skill at glossing over his changed views—help explain the disparity in media coverage. The underlying problem, though, is the media’s authenticity fetish. Reporters should of course fact-check false and misleading claims from political figures and pressure candidates and their surrogates to be truthful. Journalists have an important role to play in pushing back when candidates dissemble about their opponents or their policy proposals. But in a system in which politicians must adapt their own views to a shifting electorate, the media’s focus on discovering the “real” person behind the candidate’s public statements frequently produces pathological coverage. The straight-talking politician who always says what he thinks and never changes his mind for political reasons is a fiction. In a democracy like ours, true honesty is the price of representation.