They own the DUCK.CO domain, but use it as a community forum. I think they should use that one as a shortcut to their main product, but probably they don't want to cause confusion.
The funny thing is that the registrant of DUCK.COM appears to be Google when you do a whois lookup:
I remember reading about it on TNW that it came from an acquisition. While most of the domain names Google owns don't redirect to google.com, this one is an exception (you'd wonder how it relates to search in any way). Google wants some free traffic from its little enemy?
Actually, the use case is more for someone like me - I remember that I want to try DDG once in a while, so I decide to visit it, and then I can't remember the shortcut and am annoyed that I have to type in the whole name.
Obviously serious users of DDG don't have any problem with it.
Actually, you can already use www.ddg.gg, which redirect you to the HTTPS version of DDG.