When Binance first start it didn't have highest trading volume and IEO yet. What they had at that time, easy to use and friendly website designed, trading terminal, enthusiast support team, good strategies. I remember that time Polinex and Bittrex were biggest exchanges, they refused my KYC dozens times, while Binance completed it in a day. Because of that, now Binance is the biggest exchange.
Binance KYC is the best here. It's almost instant as long as requirements are met. KYC verification isn't also compulsory, it's only needed for increasing daily limit and withdrawals of fiats