How many times in the history of the publishing industry and writing has an individual sat down and said to him or herself, "I want to write a book! What shall I write about?" It might seem humorous, but is a valid, and important question. Conventional wisdom is to write about what you know. Fair enough.
A more important question is will the book have an audience and will I be able to sell the book to that audience? Why write the book if it will never be read?
Fiction seems to be much more difficult to write and successfully market and sell than non-fiction. People want to be entertained, but they also want, and need, information and learning about topics of interest can be just, if not more, entertaining than fiction. For authors who desire to write a non-fiction book, finding a niche to write about might be the most important bit of research the author conducts before the first sentence is written.
Why target a niche and not the population at large? Would a general book on a broad topic have the potential for more sales? Numerically speaking, yes. But how do you market such a book? The competition from so many other factors make it harder to grab the attention of the general consumer and convince that individual to purchase the book.
A niche market book is different. The niche can be identified, quantified and located. The people who are interested in this niche topic can also be identified, quantified and located. This makes marketing much easier. The message can be tailored and carefully crafted to fit the audience. The brand can be carefully identified with the book and the niche.
Targeted marketing and sales efforts get better results and rate of return on investment than a broad, scattered and haphazard approach does. Why waste time, effort and money in an attempt to get the message about your book to someone who has no interest in purchasing it? Wouldn't those limited resources be better utilized by getting the message about your book directly to the people who are interested in the topic you have written about?