Thing is, bangs are for people who don't use launchers (Alfred, LaunchBar, QuikSilver, Launchy, Gnome Do, Unity Dash, etc.) with web search functions. To redo a search for me: Cmd-Space, Up, Return. To add a bang to a failed DDG search: click in a search bar, make sure the whole search text isn't selected so it won't be replaced, remember which bang I want, type it in, return.
Those who don't use launchers outnumber us greatly, of course. But by virtue of being able to specify an alternative search engine using the same efficient keyboard commands as everything else, bangs aren't much more than a pain in the butt.
Even putting launchers aside, not every DDG user is going to use bangs to get results from another site. I'm sure many, as I, have the muscle memory to bring up google.com in the flash of an eye, whereas awkwardly clicking in a search bar to type in a bang surely can't be used by even the majority of DDG users.
(Of course, I don't have data to back me up, but say DDG became as ubiquitous as Google, this would be unquestioned. All the users' habits in organisations where Qwant is set as the default could make an interesting data set — except, of course, it wouldn't be possible to track who used what)
tl;dr: Bangs are too inefficient and usage must be too inconsistent to use as the basis of improving the results.