In order to say "yes" to a writer's nonfiction book proposal, every publisher needs to see three things:
Your concept is the--fresh, unique, compelling--premise of your book.
A publisher is looking for a book with a unique and compelling concept. The best books serve readers by presenting a timeless truth with a vibrant fresh angle. When publishers read these proposals, they say, "Hmmm...I haven't heard it put quite this way before."
Your platform is how you're reaching audiences.
You must demonstrate that, as an author, you are building a solid platform and is already reaching audiences.
This might be reflected in social media numbers. It might mean you're reaching audiences as a speaker. Or perhaps you write regularly for a popular online magazine.
Although there is no magic formula, an author does need to be reaching readers.
Your writing is how you organize and express your unique concept.
Your writing captures the reader's attention by engaging him or her with well-organized and beautiful writing.
Could missing one of these three be a deal breaker for a publisher? Unfortunately, yes. While a publisher will occasionally take a chance on a brilliant writer with very little platform, you make it easier for a publisher to say "yes" when you can demonstrate that you are building a platform.
If your book proposal demonstrates that all three of these elements are strong, it will be hard for a publisher to say "no."
If your proposal lacks one or more of these, it will be hard for a publisher to say "yes."