The interface is part of the reason -- it's easy to navigate, and conversational threads are clearly laid out. There is an alert icon that tells you if someone has replied to one of your earlier posts, something missing from many social-media sites. You click the alert icon, and all the replies are listed in chronological order, simplifying the management of multiple conversations.
The public "font page" display is also intelligently laid out, with posts that have been read and upvoted ranked according to popularity.
And there is a voting scheme -- although controversial, the voting "karma" system allows the system to rearrange posts by popularity without any effort or intervention.
The site is broken into forums, and users can create a new forum if they care to.
I used Usenet for many years before there was a Web, and Reddit reminds me in some ways of those days (but it has a better interface).