Okay s btc price selling for usdt on bittrex is about 500 dollars less than on gdax/coinbase/gemini etc. If thats the case, why would anyone sell bitcoin on bittrex?
first of all $500 is not called "so much lower" it is only 3% lower than other places, you are talking about $500 out of nearly $16,000!
secondly it is a USDT market not USD market. people are not really using tether specially on other exchanges like bittrex. main usage is from bitfinex and poloniex.
and finally bittrex has been scamming people lately, and obviously many have left that place and that always leads to less traders, less money and fewer people buying different things.