It might have to do with there being real identities linked to them now from Google+, which many say was the main purpose of the project; not to mention the improved recommendation engine and upvotes, etc.
Facebook insists even more on having "real people" using their real names, often forcing them to vet themselves, yet at times it easily steals the crown in terms of "really bad user interactions" from YouTube comments.
Very likely much more related to the topic under which people are commenting: Non-political feel good fluff does not allow for much friction to be inserted, thus the tone of comments will be much more civilized and friendly.
But anything that goes into controversial, and particularly political, territory is bound to quickly escalate into a complete shit show, on YouTube and Facebook alike.
Which fits neatly into common communication strategies like focusing on similarities rather than on differences.
But the migration of YouTube channels and comments to G+ linked identities was reversed[0], wasn't it? Not to mention that G+ has since been shut down.
Shouldn't that make the abortive effort irrelevant for the quality of YouTube comments today?
[0] https://googleblog.blogspot.com/2015/07/everything-in-its-ri...