You’ve built your career to be an expert in your field. You know all the gains you’ve made and what you’ve accomplished, but you need to lay it all out in your proposal. The more about your platform you can include in your proposal the more marketable you’ll seem to agents and publishers. Agents and publishers see proposals every day, so how is your expertise something that’s a bonus to you and the project?
It will show the gap in the market.
How are you going to be unique and say something different? By showing us that no one else has done this before and if they have why you are going to do it better, more comprehensively, or with a different angle. If you aren’t saying something unique, why would a publisher want to publish you? Publishers have experts in many field writing books for various non fiction imprints, how are you going to crack that list? By doing something that no one else has, or if they have doing it more uniquely with a fresh angle.
Unlike fiction authors, you haven’t finished your book yet.
Fiction writers can show their voice and style through the manuscript they submit, but non fiction authors sell books on proposal so you need to use your voice in your proposal writing and sample chapters. The more comprehensive your proposal, the more your voice shines through.