Because it won't work in every browser and making shortcuts the same across Firefox and Chrome is a hassle, plus you can never get up to thousands of shortcuts that DDG has.

Sometimes, I'm just experimenting. I had no idea that DDG had a Flickr bang because I never had the need to use Flickr before, but I just added !f to my query and I got the results I was looking for.

I use both, but the '!bang' syntax is more convenient when you simply want to repeat a search using a different search engine, or want to use an engine you don't have a quicksearch for (you could of course add them, but sometimes it's just not worth the trouble for the few times you need it).

DDG is my new-tab page, so that helps. Plus, if the wiki page is lacking, I can just go back and do a normal search or a Google search or a Stack Overflow search just by changing a couple characters at the end.