Ramesh Ponnuru explains why for National Review Online.

He wanted to be the liberal Reagan, or rather the liberal anti-Reagan: the person who pulled American politics back to the left a generation after Reagan pulled it to the right. Bill Clinton had not done that. Instead he had governed in Reagan’s shadow. Obama thought that the time was ripe to emerge from that shadow. Many of his supporters wanted the same thing. For them, “Yes we can” — one of his 2008 campaign slogans — meant “Yes, we can overcome Reaganite conservatism and Clintonite triangulation.”

As late as last April, the commentator Fareed Zakaria was able to write that Obama had pulled it off. “Obama aspired to be a transformational president, like Reagan. At this point, it’s fair to say that he has succeeded.” This judgment was not crazy, but it turns out to have been premature. Less than a year later, it appears that the Obama project has failed. …

… Reagan left office only slightly more popular than Obama is now. But Reagan also left his party holding more seats than it held when he was elected. The reverse is true of Obama, at every level of government. The public was also much happier with the state of the country when Reagan left office than it is now — although that measure of public opinion may say as much about growing polarization over the last few decades as about Obama’s performance.

The most important difference in their political records is that Reagan was followed in office by an ally while Obama will not be. George H. W. Bush ran for Reagan’s third term in 1988 and won a decisive victory. In this way he showed that Reaganism’s electoral success was not dependent on the surpassing political talent of Reagan himself. If she had won, perhaps Hillary Clinton would have shown that the political trend lines Obama had identified could carry even someone who lacked a trace of his charisma to victory.