I think there are two factors at play here:

1. Steemit is still very small. Only ~50k accounts. So there is a strong sense of community. Compare that to, say, Twitter, which has over 300m accounts and little sense of community anymore.
2. Steemit incentivses good behaviour, and being polite and friendly to each other. I suppose trolls will come here eventually. But it's only the polite and friendly posts and comments which will get upvoted.