Michael Kazin of Georgetown explains why at The New Republic:
This might seem odd, given Americans’ long romance with wealthy entrepreneurs and the enterprises they build. But a talent for developing private companies and making big profits seldom translates into wooing a majority of voters or governing a contentious republic. It may, in fact, blind one from recognizing critical differences between those equally difficult endeavors.
The most famous example of this disconnect was Herbert Hoover—a multi-millionaire who, like Romney, believed that America needed a shrewd capitalist at the helm of state. By the age of forty, the dour Quaker from rural Iowa had made a sizeable fortune as a metal engineer and developer of mines in several foreign countries. During World War I, Hoover employed his skills for a large, humanitarian purpose, arranging for food to be funneled to the millions of Europeans impoverished by the war. Then, in the 1920s, he became a high-profile Commerce Secretary, bringing industries together in trade associations where they could regulate themselves. Thus, Hoover had gained fame as an unelected public servant as well as one of the richest businessmen of his day—in contrast with William Randolph Hearst and Henry Ford, self-serving contemporaries who had earlier flirted with presidential runs.
The obvious counterexample (refutation?) is George Washington, was a (wildly) successful businessman who was also a successful president.