For the following reasons:
A. Both are free for the user but Google provides consistently better quality results. DuckDuckGo is the underdog in this field and it is difficult for it to become more popular than Google given that the results it provides are less efficient than Google’s. This is the same problem that Yahoo and Bing are facing: they are not able to match Google’s quality of results and for this reasons users — always in a hurry and hating wasting time on useless pages — keep using what they know works best.
B. People tend to underestimate how bad is the influence of these websites tracking everything we do online thanks to Google’s cookies system. Until our eyes are not opened, this is considered an irrelevant issue, so DuckDuckGo seems to have no advantages (its non tracking feature being irrelevant in this phase) and only disadvantages (lower quality results). Once we open our eyes, it becomes horrific what these website like Google and Facebook are able to do (and actually actively do!). But this eye-opening moment happened only very recently (with the Brexit referendum or Donald Trump’s election for example) and people have only very recently been made aware of how these companies exploit the pervasive tracking of their users.
The advantage of DuckDuckGo (its non-tracking features) has thus only recently come to surface and we will need to see if people are willing to change their deeply ingrained habits for the sake of protecting their privacy.
A big big change would be triggered if Apple would remove Google standard search feature from their iPhones and iPad (knowing that for this feature alone Google, quite understandably, pays Apple 1bn USD a year).