That's because they both allow for creation of very segregated sub-groups that don't inherently interact. You won't see a lot of porn on /r/all because Reddit d

That's because they both allow for creation of very segregated sub-groups that don't inherently interact. You won't see a lot of porn on /r/all because Reddit doesn't want you to, even though at least 50% of Reddit is porn, it's just bad for business. It's basically a "mini internet".