It takes more than technical skill to build a business; it takes knowledge of how business works.
Michael Gerber, in his business classic “The E-Myth,” coined the term “entrepreneurial seizure.” (Note: if you haven't read this book, stop now, go to the library or bookstore and get it. I'll wait here for you while you read it.) As a refresher, Michael refers to the entrepreneurial seizure as an event that occurs when a technically inclined person decides that they know enough about their craft to start a business.
Mr. Gerber tells the sad tale of a pie maker turned pie businessperson. Her friends told her she was so good at making pies, she'd be crazy not to go into the pie business. I'll let you read the rest of the tale. In the meantime, let's look at a few other causes for ES among contractors.