Because Poloniex didn't add it.
The rumors tell that Poloniex will add it only after the full nodes will work.
Until then you can use Bittrex.
Yes I also bought and still save waves on bittrex. I think waves is one of good altcoin, the prices always increased some today and if sell now I will get profit about 30%. I sure that waves will available on poloniex maximum in this year.