Ensure that Backdrop is listed as a supported CMS in all locations on civicrm.org
There are a few places where the CMSes that integrate with CiviCRM are listed, but Backdrop is not included. I've found these examples so far, but I bet there are others!
- https://civicrm.org/explore-civicrm, under "What is CiviCRM?"
- On the "Stunning websites that promote your members" slide on the homepage; I know that the linked 2015 blog post itself predates Backdrop/CiviCRM integration and therefore doesn't mention it, but the slide still can!