A successful entrepreneur such as Bill Gates picks a single line of business and pursues it with devotion and energy. Some can focus on more than one thing at a time, but for most, to divide the focus is to risk failure. An academic specializes. Most of those who work in more than one area risk being accused of intellectual flightiness, are taken less seriously than they should be, and fail to climb high in their fields. Our Carl Sagans are few and far between, and even they must struggle for their proper professional due.
Writers also specialize, concentrating on mysteries or textbooks or biographies.
They build followings that way, and their editors know how to label them for sales purposes. If a mystery writer insists on doing science fiction, he or she often adopts a pseudonym for the purpose. Even Stephen King did this, writing mysteries as Richard Bachman before he grew so famous that his following would follow him wherever he went. My own specialization is a little different. My work over the last thirty years has fallen into clusters of fiction and nonfiction, each cluster centered on one of those themes.