I do not understand it, Dogecoin belongs among top coins with high trading volume, the higher volume, the higher profit exchange has. So there is no rational reason why they haven't done it yet.
One thing that for sure one of the reasons why Bitfinex exchange ignored Dogecoin because it does not have a fixed amount and the amount will continue growing because this coin was can be mined. There are a lot of exchanges accepting Dogecoin but all pairs that dogecoin have are in red color, just my own point of view.