From what I understand, it's mostly Bing + Yandex (for Russian pages) these days - the additional sources are websites that submit their own custom indexing of their pages.
I suspect the nature of things just leads to that even if they have multiple big upstreams.
If bing offers up the best results most of the time DDG will, as a result, offer up bing results most of the time. "Best results" easily determinable even with DDG privacy constraints.
They're definitely doing some cool stuff at DDG and its certainly a hell of a lot more than "a wrapper around bing." I just don't think expanding their own index infrastructure is something feasible for them right now.
Sure, but I don't think it's fair to compare DDG to other search engines that crawl and index the entire web themselves. It's a different user experience for sure but the indexing is (for the most part) the same as Bing.