Most people when it comes to selecting an exchange to trade on don't take into account the fees associated with each trade that much, simply because most people don't trade enough volume for that figure to have any sort of major significance.

Bittrex was extremely popular. But it's definitely not as popular now, especially after legacy accounts were removed from having the rights to deposit and withdraw without ID verification. That's why you see huge drop offs in volume in Bittrex as of now compared to a year ago.

Thus, it's probably wrong to assume that Bittrex is still that popular as most of these traders who want to remain anonymous are moving towards Binance where a 2BTC threshold is kept for anonymous accounts. For the remaining account holders, though, they're most likely there to trade very specific pairs not offered on other exchanges; or simply diversifying their trading across multiple platforms.