As an example of this that can be demonstrated right now (and potential insight into why this is the case), search for "HTTPS tutorial" in DuckDuckGo[1].
This search is expected to return results that provide a tutorial on the implementation or usage of the HTTPS protocol. The first three results seem decent enough, then things get weird. What follows on the first two pages or so are (in order) tutorials for: Ubuntu, PARCC, HTTP (relevant), VPN, React, R (language), Windows 10, Shiny (related to R?), Matplotlib, scratch.mit, AutoHotKey, Quickbooks, Node.js, Java, Python and Kubernetes. As far as returning results for HTTPS tutorials DuckDuckGo has seemingly not done very well. Many of these results are related to software so one would expect these sites to have well-executed SEO. It looks to me like DuckDuckGo has confused the "https://" in the URL for an indicator of content related to HTTPS. But does Google's search algorithm do any better?
Try the same search in Google[2] and it seems to have similar problems: if you go several pages deep the results still do not acknowledge that "https" is missing from the actual content of the result. But the fact that Google is used more frequently has allowed relevant results to bubble up to the top because people click on them. Only one result on the first page does not pertain to HTTP, HTTPS or SSL. Google's advantage seems to disappear after the first page and that makes sense because anything beyond the first page of Google search is rarely clicked.
I don't see DuckDuckGo's problem as being one of how it searches but rather its lack of usage. Maybe if we use it and talk it up to people we can work our way towards a powerful search engine that respects privacy. I could put up my own search engine and try to do better but I would be a decade behind DuckDuckGo's name recognition and that much farther from solving the actual problem.
1. https://duckduckgo.com/?q=https+tutorial&t=ffab&atb=v144-1&i...
2. https://www.google.com/search?client=firefox-b-1-d&q=https+t...