To me there are a number of factors:
Not all threads are serious although have clear BA connections.
The people are generally friendly.
It is easy to make 'friends' through attendance at a do.
There is a wealth of knowledge and many threads are actively backed up via the wiki.
The dashboards and guides are comprehensive. Regularly updated and maintained.
There is a large team of moderators and ambassadors who are genuinely welcoming to new posters. Some forums you get your head bitten off as a new poster asking a question.
I'm sure there are many other reasons.