I believe it's their name (and domain) that is holding them back. Sure, Google is a silly word but it's now ubiquitous. But "Duck Duck Go" simply doesn't roll off the tongue. It will make adoption much harder.

The shame of it all is that they do own duck.com. Why not rebrand at this point?

So instead of answering questions by saying "why don't you just google it" people could suggest that you someone "duck it".

Hmmm, maybe that has problems of it's own...