Except that CSN Philly is on lots of cable systems all over eastern Pa and new jersey. As well as fios. Price is established.
That doesn't mean it isn't low enough for DirecTV. Also, those cable systems are not directly competing with Comcast, only FIOS is. And FIOS is not yet available in the entire city either, and won't be fully covered until 2016.
DirecTV would be a major Direct competitor however, and immediately in the whole DMA of Philly.
The price may be established, but they may still be asking for more than DirecTV is willing to pay. Also, Mike White has said the current focus is on retaining customers, not gaining customers. Adding CSN Philly isn't a real priority in that regard.