The closest Philly had to 24/7 was back in the late 90's with Tri State Media News:http://www.youtube.com/watch?v=75vOCxp6sL0
and WTVE's attempt at Philly TV News which occured at the same time.
They both failed, and Comcast absorbed TSM for CN8. CN8 wasn't so bad and they had a 7pm 1 hour newscast, during CN8's heyday. People from Northern NJ to Baltimore were exposed to news not concering them from the Delaware Valley.
Their 10pm newscast had no chance locally going against WTXF and WPHL (which had it's own news-not produced by someone else) and WFMZ for those that want news from that area.
Unlike Time Warner and Cablevision that like doing local cable news, Comcast has felt local cable news isn't for them. Cablevision uses it an advertising point against the satellite companies. Perhaps because Comcast has had CSN exclusive, they never felt they had to put any more effort to keep cable subs from satellite.
The other issue is none of the O&O stations want to have 24/7 news stations as a side business. Comcast owns majority of the cable systems, and any company that wanted to launch local news would have to secure good channel placement. I don't think WTVE secured decent channel placement at the time, and was treated like a rimshot mustcarry station that Comcast didn't want to carry in the first place.
In DC, Allbritton not ABC owns the ABC affiliate and wants to feel more than an affiliate still operating a newschannel in a world where they are less and less important everyday. Rather than airing a 4pm newscast, they are failing miserably with Anderson on at 4, so that the newschannel would still have some relevance as an alternate to WJLA.