Ok, BIG question? why is this forum so successsful, it had a wee FB stint, when thing went a bit T.U., but that was closed down.
I think ...
There is a shared vision/expectation RE the forum.
There is a good mix of established users but also welcoming to new people.
Generally the forum benefits from being a forum used by people who already have experience of forums and other cycling forums. You get very few "this is my first time here" people and hence you don't have the reams of "what should my first bike be?" type posts (that has a whole sub-forum to itself on the CTC forum).
It has good design, if you want to get ranty there's a space for that, if you want to get in-your-face or political there's a space for that. There's also a space where that won't happen and you can just talk about hub gears, heat pumps, or whatever.
It's also maybe worth thinking of other cycling forums that did not do well
The CTC forum was much better than it is now because half the established crowd left when they became CUK and then more left because it was empty.
The C+ forum fell apart when they rebranded to bikeradar, which I think was a loss of identity issue.
LFGSS and BHPC went facebook, which suited their users.
BROL is still going strong (I think, I could just be looking at archived posts though as I mostly use it for reference not discussion).