I don't think that anyone still cares a lot about the anniversary of that site and that's why no one is celebrating its anniversary. Like @Poker Player said that the forum is bitcointalk.org not bitcoin.org and that's a valid reason for the members of the forum to not celebrate 15 years anniversary of that site. I also believe that the site is very important in we go back in the early days of Bitcoin, but now a days no one is that much interested in celebrating its anniversary. I hope you got your answer and if you are satisfied with your answer then kindly lock this thread.