As a matter of fact, not everything has to be about mainstream audience, it depends on the product.

http://scholar.google.co.in is not meant to be a mainstream product.

Where as http://google.com/search is a mainstream product. And DDG thinks itself as Google's competitor with a heart for privacy, that's their USP.

So yes, as far as DDG is concerned, mainstream matters. It's a generic search engine, not a vertical for scholars or the like.