It's always been possible to designate an image to the public domain on flickr, by pasting a designation into the description, but this gave it a level of overhead that I think discouraged people who hadn't thought about it much to avoid it.
Edit: This is getting downvotes, which I'm not sure I understand, but I'll assume it's because I didn't explain the problems with public domain designations. If there are other reasons, I'd appreciate a comment explaining them.
IANAL, but as I understand, there's some debate over whether works can be committed to the public domain at all. This makes some individuals, and a lot of big companies, hesitant to make use of anything with a public domain designation. Most people who try to commit something to the public domain rather than picking a popular non-restrictive license do so in effort to remove all obstacles to people using it, without realizing that there might be more obstacles with a public domain designation than a CC-BY license.