One thing working to Twitter's favour is that many players and coaches are there and forums generally can't offer that. In the case of the NBL and Twitter, the refs boss, the execs, administrators, players and some coaches are all there and reasonably responsive. Same with media - you can get a public response from Boti or Roy Ward quickly.
As for Derek, I've always assumed that he's lacked the time to nurture the site. It's often hard to find time outside regular work and life to further a niche community, even when there's potential return from sponsorship and ads. I've had a load of plans for Hoops for years that I've never got around to. I actively moderate in the areas I follow, but there is always room for Hoops to get bigger and better, especially in encouraging those interstate to participate.
Once you have a decent community, sometimes it can just roll on for years without too much of your own effort, but you need new users coming in frequently and if OzHoops closed off registrations, that would've hurt. It's a limitation in one way for Hoops, but here a newcomer can generally come in and contribute from their first post without even registering unless they're on a particular IP class, etc. I imagine there'd be a lot of people who would never have started contributing otherwise.