The biggest reason by far that nonfiction ebooks don’t sell is simple: the writer is targeting a small audience. Use KindleSpy as a tool to assess the size of the audience for your proposed ebook.
For example, I just ran a search for “nonfiction ebook” on the Kindle Store, and according to KindleSpy, the average monthly revenue for this topic is just over $1,000.
If you conduct a search and the average monthly revenue is low, you’ll need to tweak your topic if you want to increase your revenue.
Don’t have KindleSpy? (BTW, I have no connection to the developer, I just use the tool and find it useful.) Use Google. Enter your topic into Google, with “ebook” tacked to the end of it. Depending on the topic, you may see some AdWords ads on the right sidebar of the search results. Ideally, you’ll see some advertising there. You can also enter your topic with “ebook” into Facebook’s search query, to see whether people are advertising there.