Why write a niche book?

Writers tend to think first about just writing a book. Most of them don't think about marketability and business. If they do think about it, they don't consider the broader demographics and factors. When you write in a niche genre, for example, it's already going to have a narrow audience. A narrow audience though is relative when it comes to sales. What does this mean?

• A narrow audience is like a narrow river. You have about "this much opportunity" in that space. It flows and people come in and out (your audience and customers). 
• You probably don't have that much competition in that space, because it's not a common theme or idea you're writing about.
• You do want to attract people from the narrow audience to your book.
• Because you don't have much competition than your book might be one of a handful or the ONLY one on the bookshelf to select.
• If you're one of a few, your chances of capturing those few buyers goes way up. So fewer consumers, but consumers who are serious about wanting to know about your topic won't have a bunch of choices, which leaves yours.
• Capturing that niche market equals sales, and sales that are consistent and ongoing because there is no other book to lure away your customers.
• People are always interested in certain topics, so they will keep buying your book over the long run. 

The net result is long-term, ongoing, consistent sales. Your sales might not be huge (could be though), but a long-term passive revenue stream is a good deal. Work goes in on the front end and you enjoy passive revenue on the back end, which is why I never dissuade authors off good niche books.