If you write a book to help parents and kids deal with something everyone experiences (like bedtime), not everyone who experiences that thing will buy your book. This is obvious, right? Some of them will buy your book. Not all of them.
Good. So the next thing to realize is that if you write a book to help parents and kids deal with something only a few people experience (like the death of a loved one, or synesthesia, or satanic ritual abuse), not everyone who experiences that thing will buy your book. Some of them will buy your book. Not all of them.
This is what publishers call an "issue" book: a book for a particular situation/problem in readers' lives-- one which does not affect all people.
Because the audience for such books is narrowed by the number of people who are affected, and further narrowed by the fact that not all of those people will buy the book, "issue" books have very limited sales potential, and thus very limited appeal for publishers.
You may feel you are doing a public service in writing a picture book about little Samantha's ageusia. You may be frustrated by the unjust lack of books your child's kindergarten teacher can use to explain to the other children that when Timmy beats up on them, it's really a just another of the ways God makes us all special and different.
But publishers have warehousing costs, in addition to many kinds of overhead. They are in the business of providing only those public services that serve more than a tiny fraction of the public, and only those services the public will pay for.
Thanks to Mary O'Dea for the link!