I reckon Circle's purchase of Poloniex should have cleaned the exchange of cryptocoins that have questionable histories.
I have no ideas. They listed a lot of cryptonote coins. Almost all even total scams like Quazarcoin. all of them were delisted latter on. Only Monero and Bytecoin are left. I think Ducknote was delisted last only 2 delistings ago, Boolberry one delisting before. I do believe BCN will be among next rounds of delistings on Poloniex.
Why all were listed in first place I have no ideas. Maybe one of owners of Poloniex had strong faith into cryptonote protocol. Many did in 2014. Not that today are less believers. But today are just more people in crypto overall.