Reddit really 'dug' (see what I did there) its own grave by aggressively catering to casual users, in order to drive signups and grow the user base. I can't remember exactly what happened at which point, but it all follows the general pattern of appealing to the mass market. E.g. the curation of the frontpage via default subs such as /r/AdviceAnimals that don't really contain meaningful discussion. There's no real need to do this if one is concerned with maintaining an existing community, or the standards of it. As a counterexample, private trackers have been rather good at this, generally by having interviews/invitations for new memberships, or even disabling signups entire. Though these are really the most extreme options.

The other main problem is that like Digg, that there is a powerful cabal of moderators in control of most mainstream subreddits, including those concerned with serious topics like news and politics and they do a lot of "curation" of content by removing stuff, rather than letting the users decide on which links and topics are relevant to discussion (by, ya know... upvoting/downvoting) with chilling effects on those respective subs. That this was allowed to continue, without overhauling and maybe even democratising the moderation system of default subs, was Reddit's choice. I can only suspect that this either (a) plays into the hands of advertisers directly via corporate censorship or (b) indirectly, by relegating content from users who fall outside of the "mass-market" demographic to niche subreddits or other websites.