One of the reason is segwit getting activated widely and due to which the congestion in network getting fully reduced.Another reason is professional new exchanges entering the market to break the monopoly of some exchanges like poloniex,bittrex,kracken which almost had a lion's share of bitcoin exchange market.The bigger exchanges are facing a severe competition from newer exchanges which are ready to provide a good customer service unlike poloniex and bittrex which were well known for their poor customer service.