It's due to the way the games are provided to DirecTV by the respective league offering the out-of-market package. Both satellite and cable systems offer MLB:EI, so the league broadcasts their own set of feeds to each of their distribution partners offering the package via their own transponder space. So cable companies and DirecTV get reception hardware from the league to be able to pick up the MLB:EI games in HD.
Satellite transponder space is limited, so what DirecTV typically does is take the RSNs they already have uplinked to the satellites and authorize MLB:EI subscribers to be able to tune out-of-market RSNs while the game is taking place. DirecTV doesn't currently have an agreement to carry CSN
hilly, so they use the feed they get from the league just for those games.
The NHL does the same thing with Center Ice, but they only have the capacity to distribute the content in SD. So normally you get to watch the game on a HD Regional Sports feed that DirecTV already has uplinked, but in the case of CSN Philly they don't carry the channel, and the only feed they get from the NHL is standard def.