Every industry has its forum or community for discussion, Bitcoin will eventually propagate as long as there is the internet there are hundreds of ways to communicate social media are considered a public forum, since the community is growing when Bitcoin was first created there's bound to have a forum or a community dedicated to it.
We have a great number of Cryptocurrency groups like those on Reddit and many more on Telegram and Satoshi Nakamoto knows what to do to promote Bitcoin and that is through interaction SMF type of forum was very popular back then and it is still popular now, Nakamoto on that time could only think of forum like this because its the one suited for his creation because he can create sections and categories and full control of everything posted.