There are lots of different kinds of publishing. Trade is the one most think of when they decide to write and publish a book. Who doesn’t want to see their book on an endcap at Barnes and Noble? Who doesn’t want to do a reading and sign copies at their local indie?
But sometimes, the books people write and submit to agents and editors are definitely not trade books. Very often, they are educational. For example, I have lost count of queries that hit my inbox beginning with “Childhood obesity is a national epidemic.” While this may be true, it does not make a trade picture book. Other examples of books that will not likely find a trade home: Explaining cancer, diabetes, or other diseases/medical conditions. Explaining adoption, alcoholism, incarceration, abuse. Basically if your main character is a doctor or social worker or adult of any kind who is explaining something, you’re writing a niche-market book.
Recently, an incredibly smart single mom I admire very much asked if I knew of any books explaining the process of artificial insemination. I said no, trying not to cringe. She said, “I guess I have to write it, then.” We don’t know each other well enough yet for me to have told her all the reasons I think that book doesn’t exist. And if she writes it, well, that’s a bridge I’ll cross later.
I’m not discouraging books on these topics. They can be very helpful to have when kids start asking questions about heavy life-stuff. They are great educational tools. But they are unlikely to be the ones pulled off the shelf for multiple reads night after night. Of course there are exceptions, but as the saying goes, they prove the rule.
There are, of course, ways to approach these subjects that are more widely accessible, one being to write a character who is dealing with said issue, without making the story about that issue. For example, I have a friend who, as a kid, found out he had diabetes and had to start taking insulin shots. But he was also a smarter-than-you Thrasher-reading skater/alter boy whose holy grail was a vinyl Space Oddity single. A story about a kid who has to take insulin shots is very niche. A story about a well-rounded character who just happens to inject himself daily as part of a larger story is going to have a wider appeal.
Niche-market books take niche-market publishers to make them successful. Do your research before submitting. Find agents and editors who know that market well, and understand that many do not.