Bittrex dropped the ball when they kept adding and delisting shitcoins each week. They didn't care if it was a scam or not. This is the period when Poloniex became number one with their careful additions. And then Poloniex introduced margin trading. It made them solidly at number one.

However, Poloniex suddenly ran into customer related problems - piling withdrawal problems. People were scared of an exit scam in the making. This was the time Binance stepped in and took away the trading volume and became the number one. Poloniex was then sold for unknown reasons, maybe to avoid exit scamming remaining users.