I wonder that why Bittrex so popular as their commission charge so high compare to Binance, Kucoin & others? Is it because Bittrex has a lot of pair to trade? Confuse... 
Popular as a "literal" popular? Or you are talking about the people's usage?
Literally, Bittrex is popular because they are here in the crypto world for long time now. Therefore, they already established a name in crypto exchange industry. In comparison with Kucoin, I don't see Bittrex is being outshined by these exchange. Kucoin is more sh*t to others compare to Bittrex based on my own view.
About usage, I don't see that Bittrex is far the most popular that being used. Binance is the most popular I will assume. As time passes by, Bittrex good reputation now havesome dirt much especially with the rules under the strict implementation of KYC although other crypto exchanges are doing the same and the most concern by any traders, the CUSTOMER SUPPORT.