I think it is much simpler than that.
This is blogging, with incentives.
Reddit already had awesome comment sections, beating the comments on Digg by lightyears, even before .self and AMA became popular. But the latter definitely helped. Imgur also has organically grown an awesome comment community.
The main focus/use of both platforms is just different. That not necessarily helped by the design either. This here is so... slashdot.
Generally, it isn’t necessarily a good idea to compare. Just check Medium... top quality content and often has only few and regularly crap comments. That bad they even felt the need to create a highlight section.