It was the first forum to discuss bitcoin that still exists today, and it was used by satoshi.
Even better: it was created by Satoshi!
Cøbra explains it much better than I ever could:
A lot of prominent members in the Bitcoin community started out on bitcointalk.org, then migrated to Reddit, and subsequently migrated from Reddit to Twitter. Many of these became "thought leaders" on Twitter, and no longer are active on Reddit or bitcointalk.org. I don't think it's because of the format or feel of the forum, I think it's more of an ego thing. The thing with Twitter is that if you have a minimum of a few thousand followers, you can write a good tweet and watch the likes and retweets pour in. It does give some level of satisfaction, something that can't quite be replicated on a forum. Eventually over time you find that people start trying very hard to be liked, so rather than stopping to form an opinion, they end up trying to fit into the group think. You see it in Bitcoin Twitter especially when everyone has the Lightning emoji in their name, the whole "UASF" in the bio/hats thing, absolutely toxic hatred of altcoins, and other examples of people trying desperately to fit into what everyone else in the community is doing. Everyone is always organizing to rally against something, and that's where the problem with these platforms shows itself, it enforces conformity. Conformists are easy to control and manipulate.
See the thing with these platforms though is that they intentionally trick you to seem bigger than they actually are. How many people on /r/Bitcoin post and contribute each day? It genuinely isn't that many, and on quiet days you can see a handful of very active discussions, and even those don't have very many unique new people. But the way the content is structured masks that. This forum has an insane amount of discussion taking place each day, across many different aspects of Bitcoin and periphery topics (economics, politics, services, etc). If you want price memes and surface level discussion, Reddit and Twitter are great, but this place has more variety and people are generally open minded, if you know which topics to post in. Twitter is also quite a small sample of Bitcoin users, for example Andreas' account, one of the most vocal and well respected Bitcoiners on social media only grew by ~60k followers in the past year, nothing compared to the type of numbers this forum achieves with new users.
There will be many forums and many places to discuss Bitcoin, many social media sites and many Facebook groups. But how many of those will have been something Satoshi created? A hundred years from now, nobody will give a shit about Twitter, Facebook or Reddit. But there's a good chance they will give a shit about Bitcoin (and one can hope they will be using it). You can be sure this forum will be in the history books. Little kids will look over archives of our threads as they dream of one day owning 1 BTC. Threads like "we are the new wealthy elite" will be discussed in classrooms. Homework assignments will ask kids to go and research how the HODL meme originated. If Bitcoin does take over the world, you can tell your grandkids you were in the place where it all began. We have something that can't be replicated elsewhere: history.
And communication 
that's sick 
