This was discussed previously, and I think that the biggest difference is that Bing and Google are "silly non-specific" while DuckDuckGo is "silly specific", which means that when you think of it, it sounds kind of silly, and you know why and you have a mental picture of a duck or whatever in your head.

Google is kind of a weird word, but it didn't feel quite as silly even way back when it came out. Same with Bing.