Main reason: concerns about tracking and privacy.
Secondarily: politics.
DDG so far has been acceptable. As long as they keep their political opinions to themselves, the honeymoon will continue. My love affair with Google, on the other hand, is over. :(
No advertising, strong privacy policy, based in Australia (a country with a decent privacy track record that we know of).
It's $3/month which is a decent price for liberating a lot of data from advertisers.
The spam filter is playing catch-up to Google so that's a bit of a shock at first, but you can train it up well or use second layer measures like Sanebox.
Disclaimer: DDG staff - but personal opinion.
When DDG shows an ad, is there no tracking involved in that? For example, if I were to click on an ad (after turning my ad blocker off), does the destination site not get any information about me or specifically what I searched for?