You know how cryptsy was taken down ? Updated wallet with exploit inside which costed many users their money.
All depends what is more important for exchange owner safety or one week volume spike.
Exchange owners they don't need rush in adding coin to list.
Well at least that's the official version, haha...but yes Poloniex and Kraken take their security very very seriously and they can't be adding any hot new coin in a second unless it's ETC (lol). They audit the code for days and weeks to see if everything is ok.
That's why Waves hasn't been added yet and probably never will, it's not open source.