I guess what's not working for DDG is that it's a me-too product. It boils down to this: What does DDG have that Google doesn't?.
Me-too products can perform reasonably well if they are launched by corps with deep pockets for obvious reasons(like G+). Otherwise, a company like DDG has to have something different and unique to offer.
Oh, btw, that privacy hullabaloo works only for a very small percentage of the users. Most don't care or ignore.