It's just that not even torrents - perhaps the most decentralized products that exist on the internet - are really decentralized. 
Torrents with Magnet links are as decentralized as it gets: one link, that can be shared on different websites and any other way you want, contains all the information you need to connect to multiple different trackers. The trackers will connect you to other trackers, and to other people sharing and downloading the files you want. I don't think there's a limit to the number of trackers, seeders and leechers, so it's pretty decentralized.The problems become evident when older, less popular torrents no longer have seeders. I've seen torrents with many incomplete leechers and no full seeders. If that happens to your decentralized forum, it basically ends.
They are also banned by many home ISPs so even though this is already an impractical solution to host a forum or perhaps even a BBS (if you want to go retro), there are also legal implications to this as well.