Because Comcast in Philly uses a ground based distribution system for thier signal. The other Comcast Sportsnet's use satellite to distribute. The rules/law says if you use satellite to disctibute you must make it available to other providers but if you don't use satellite, such as using fiber, then you do not need to share with others.
And even if it were available to Dish, it wouldn't be willing to pay much for it. Every Philly sports fan who would treat CSP as a make-or-break channel has already found it elsewhere.