Well all exchanges need to take care their reputation, they need to maintain their credibility to maintain the volume of traders coming to their platform everyday. This only be happen if they got a good coins to be traded, which traders will assure to have buy and sell every minute. Then coins delisted came out, they need to eliminate those bad coins which make their exchange looks dead.
Right mate because traders can be lured with the exchange good reputation and this can be damage by shitcoins or non active currency,thats why some exchanges need to delist thos inactive coin and may listed again upon activity depending to the dev of the said coin and the decision of the exchange company.this has same issue with Coinmarketcap in which some coins need to be delisted