poloniex and bittrex are both silent for a long time 
The two exchangers have started to be replaced by Binance or Okex or Huobi. Many investors prefer to trade with exchangers such as Binance or Okex because of the greater transaction volume and also the security of their funds.
What do you mean by started? Man, they have been replace a long time ago.
we can see on the ranking https://coinmarketcap.com/rankings/exchanges/
Polo and Bittrex have only over $100 million combined trading volume, while Binance currently have $7 billion, number one in terms of trading volume in overall.