From the YouTube Help Center:

"You'll soon be able to comment, upload, and create channels without Google+. The comments you make on YouTube will appear only on YouTube and not also on Google+ (and vice versa). Check out our blog post for more information and keep an eye on this article for updates."

Building a comment system at the scale of YouTube (while preserving the existing set of comments), along with creating a new identity system for it is likely not a small amount of work. I'd guess that they're still working on making it a reality.