See also Why are YouTube comments so bad?
Yonatan Gershon’s answer substantially covers the differences:
Quora’s comments are much, much better. Both because of more well-thought-out methods of enforcement, and because Quora is a smaller platform upon which to attempt meritocracy.
In addition, for all that Quora likes to distance itself from being social media, it’s more social than YouTube. Following and exchanges are built in to the fabric of Quora much more than YouTube, where interactions are pepper spray. So there are less social repercussions on being an arsehole there; the absence of BNRB enforcement on YouTube is a symptom, not (just) a cause.
And, again for all that Quorans often differentiate answers and comments, answers and comments are part of the same fabric on Quora: exchange of ideas. On YouTube, the point is the broadcasting of media; it’s democratised, but it’s only occasionally two-way. The comments are not part of the same fabric, they’re more incidental. So they don’t attract much care and thought to begin with.
That said, YouTube’s much larger size would defeat any but the most numbskulled attempts at moderation.