bitcointalk could be a forum for crypto, actually it has been made for that.
Yes, but not only one. Actually there are many other crypto related forums and communities on the net.
https://altcoinn.com/ is one of these.
Yes, there are other crypto forums but the question is on how much traffic they had? how many visitors they have? Bitcointalk is the most popular crypto forum and this was the first ever made that's solely made for the bitcoin community.
But while the crypto's grows, there's a vast addition of altcoins and bitcointalk became the wholesome crypto forum through announcements, bounties and other altcoin related sources.