I think a big part of it was that it was seeded (and continues to be watered with) people who are looking for funding and/or people whose hopes are already dependent on YC and other sources of financing. People have to behave well, or there are consequences far beyond the forum.
That's why I mention intention of the creators. Reddit is meant to be "the front page of the Internet", which means it needs to be enormous and serve many different communities.
That's because Reddit has a million subreddits and to some extent they compete with each other. You may find this study of virtual conflict dynamics interesting: https://snap.stanford.edu/conflict/
