The biggest problem Reddit has encountered, is encountering, will continue to encounter is that it's an architecture masquerading as a website.

It does this because it has to: the architecture underlying Reddit requires centralization and constant hand-tweaking to function. Fundamentally, Reddit is an algorithm designed to aid and magnify viral market spread operated for the benefit of a community with a vehement hatred for viral marketing. Reddit is metaphorically a kid with Severe Combined Immunodeficiency Syndrome choosing to clean public latrines in Bangladesh. Go go, li'l dude but as soon as your bubble suit gets punctured you're fucked.

It's not even Reddit's fault. Conde Nast bought them for their architecture, not their community, then discovered the previously-mentioned scalability nightmare rendered their new toy incompatible with their old empire so they put it in a corner to see if it would sort itself out. Anyone with a passing understanding of social media would predict that a hand-coded "virulence engine" is likely to mutate into something uncontrollable if you don't tend it carefully. Meanwhile, all the effort was expended building Reddit into a "community" rather than a "method" for other communities to propagate. Think about it: If your karma and your profile didn't follow you from /r/shitredditsays to /r/realgirls, what would the problem be?

No one ever talks about the "PHPBB" community because it's an architecture, not a website. If, say, the SFWPorn Network were its own autonomous community, with its own admins, its own spam engines, its own policies and its own mechanics... would anyone care what happens on /r/shitredditsays? If /r/creepshots had to find its own host, its own admins and its own interface to deal with the FBI... would it persist?

Eventually, Reddit will fragment into unrelated communities. It hasn't yet because Reddit has even less of a chance of monetizing that than what they have now; as the Webtoid guys figured out, back when Reddit was open source, it's really tough to keep an open petri dish of agar from developing only the bacterial colonies you want.

So they keep it going until they can figure out how to make it work. np links pretty much show how far they are from accomplishing it yet how badly they need to.