Sometimes it's a lack of focus, but often it's simply a lack of self-discipline and persistence. The number of people who begin writing a novel, nonfiction book, or screenplay is huge, but most of their projects get abandoned after the first 10 or 20 or 30 pages.
Still, that doesn't mean people who have trouble with focus, self-discipline, or persistence are doomed to fail. It simply means they may be doing the wrong type of work. The person who lacks the focus, discipline, or persistence to build a Web site might be a great blog contributor, for example. (I know one self-described "ADHD" type who writes professionally for several blogs in different fields; the job is perfect for him, because he writes in 250-word chunks and can flit from tech topics to travel topics within a single day.)