I hate to be the eleventh person to say this, but it bares repeating if it gets lost: DuckDuckGo primarily uses the Bing index and search engine for most results. There isn't any set of branding that says this plainly, but that's where most results are coming from. Startpage is pretty different because it uses Google under the hood, but I definitely get different results from Google, Startpage, and DuckDuckGo with regular frequency. Whatever the secret sauce is on top matters a lot in terms of presentation. Any engine that puts in effort for a target audience will be able to acquire users.

For me, DuckDuckHack has always returned really awesome results on top of what I consider to be a fairly lackluster set of results for some searches that bing provides. If another provider wanted to step it up with extra sauce on top, I'd be more than happy to try if it makes my day to day better.