Perhaps because #DuckDuckGo has a html onion version if their search page which works on the "safest" setting level... 

Also, DDG claim to source results from their own index, plus topic-specific sites and #Bing so apparently it's not just a front-end for Bing. 

P.S. I like #Stract engine, #opensource and independent index:
https://stract.com/ 

 *of their search page...