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.
There is more sense in this with regards to OP than any other reason as to why, that would follow. I am not saying other reasons aren't concrete enough but, most of the foundation is laid on this very base. Bitcoin is the first ever cryptocurrency although, I've heard stories of some similar failed projects but bitcoin occupies the position of the pioneer cryptocurrency and having the creator of bitcoin create this forum., its only expected to be something as perfect as bitcoin itself. Also, bitcointalk had stood the test of time just like bitcoin itself and its highly moderated environmentcoupled with the way users of the forum ensure truths is uphold makes it the best place to get an understanding of cryptocurrency.