Lots of reasons, hard to sort out which one carries the biggest burden. I think the whole process really got started with the Spurs 2007 championship. The years leading up to that pulled in a number of good Mavs, Suns, and Pistons fans. It also pulled in a lot of dumbass GNSF who seemed to think that having outsiders on a Spurs board was an affront to God. (FOUR RINGS, GOT!) Its a credit to the outsider fans who stuck around through the needless ringsmack. With that said it doesn't absolve them from their part in trolling right back in the ensuing seasons as the Spurs couldn't return to glory. I'm not saying they didn't deserve it, but that was where things started to lose their way IMO. Previously the trolls would come out in response to ty takes that were out place in the forum's generally high quality BB discussions. Around this time the trolling started to replace the BB discussion. If there was a time where this place might have been put on a leash it was then. I'm not saying that would have saved it. In all likelihood the reasons for letting it go were right; throwing a bunch of rules up wouldn't have done anything to stem the tide of GNSF idiots, while the non Spurs fans who enjoyed the NBA forum were there mostly for the lax moderation compared to other sites.
Then the Lakers got relevant again and their trolls came at the exact worse time. By the time that the post Gasol trade Lakers were established in the league the NBA forum had pretty much been declared the wild west. It was mostly self regulating. You went downstairs and you entered at your own risk. Threads got derailed but with a little patience you could find some real gems, both in terms of entertainment value and BB relevance. In a short span of time we all saw how precarious the balance had been here. The forum's quality was anchored by the fact that for all the nastiness and fighting that went on it was ultimately people who were fighting to maintain something they had a stake in building and along came idiots with no such scruples and all the freedom they needed to things up for everyone. That was when the forum crossed over from 'acquired taste' to 'unreadable'. Around that time I (and plenty others) all but stopped visiting the forum much less posting there and there was little to bring in any new posters except people who were into juvenile already. To use a National Geographic kind of analogy back in the day it was a like a highly compe ive ecosystem, which naturally leads to an overall survival of the fittest; fast enough to not get eaten, strong enough to catch your own meals, tough enough to last in the lean times. Guys like Kool and Luva were like a plague introduced to an ecosystem. The survivors aren't necessarily the best and brightest, rather only the ones with a genetic disposition to resist it. The only thing needed to last in the NBA forum was an ability and willingness to slog through an endless stream of unfunny schtick repeated over and over over and over.
A few strategic pinkings restored the balance, but it's going to take some time to bounce back. And in the mean time, even with it being the offseason, I think what's hurting the NBA forum is a growing apathy towards the NBA itself. It's so easy to follow every detail of one's hometeam these days, but very little that is compelling about the league wide in terms of rivlaries or any sort of universally marketable superstar. Equally problematic is that demographically I think most of us in here cut our teeth on the NBA during at least part of Jordan's era and the story since then has been the NBA constantly tweaking the rules to make it less physical and easier to score, and thus distancing the game as it is now even further from that time, rather than connecting fans to whoever they are pimping as "THE NEXT JORDAN" every other year.