I assume because he switched to DDG for privacy reasons, not because he thought the search was better. Also DDG has some search features that neither Google nor Bing have.
For people who still like Google search more, but want the privacy, I'd suggest startpage.com. I see even the Tor Browser Bundle uses it as the default search engine (you can still switch to DDG and others, too).
"DuckDuckGo's results are a compilation of "about 50" sources, including Yahoo! Search BOSS, Wikipedia, Wolfram Alpha, Bing, its own Web crawler, the DuckDuckBot, and others."[1]
[1] http://onlinehelp.microsoft.com/en-us/bing/ff524480.aspx