I think it's probably that DDG does a better job at marketing themselves. Whenever there is any sort of privacy scandal there always seems to be some comment from their PR dept.
Personally, I am highly skeptical regarding DDG and find much of their marketing to be hyperbolic (see the duck.com foolishness [0]). For me, I feel that if I'm just going to get Bing search results and Bing ads, I might as well just use Bing.
Ddg reportedly does collect some information (?), and their core is closed source.