The search index is relatively easy, the ad marketplace is hard. DDG is likely hooked on tens or hundreds of millions of dollars in Bing revenue which is a tough habit to kick.
That shouldn’t stop DDG from using their own index instead of the Bing search API for organic search results though right?
Once you’re hooked on Google or Bing search ads for revenue, they can control everything you are allowed to do within a product via compliance requirements. It’s a clever and effective form of regulatory capture.