Bitcointalk was created by Satoshi Nakamoto, the founder of bitcoin. Bitcointalk is the first kind of such forums. Bitcointalk has most quality posts. The reason the traffic on Bitcointalk is 10 times more than any other crypto forum.
This forum is way popular than the other you mentioned, remember its Satoshi who started this forum and thats why many new crypto owners come into this site to announce their projects either for publicity or to attract investors even ETH has a topic here.
I think why bitcointalk forum is the best among all , it is the first cryptocurrency forum that is well organised. Other cryptocurrency forums are learning from bitcointalk.org forum and imitating the standard of it. Bitcointalk forum is the oldest forum for other forum to learn from.