How much of that is due to the (relatively token but non-zero) membership cost? I subscribed there years ago because I enjoyed the conversations and found it to have a good signal-to-noise ratio.
Granted, this is subjective to some degree as what I consider "noise" might be considered "signal" by others. I think that it worked because their intention was to make the sort of community that they wanted whereas Reddit seems more interested in making a platform for everyone.
I can see the use for each approach and I still find it one of the best things about internet communities in general. If you want something different, it's relatively easy to take a shot at making something more to your liking.
I just think that same $10 or whatever that it costs to join the SA forums and motivates most members to avoid shitposting would also present a barrier to the sort of broad, mass appeal Reddit is shooting for.
The somethingawful forums? With the exception of the forums expressly for bad discussions, every thread has high quality content.