The Eagles (who account for most Philly sports fans) are on Fox, so people get them OTA, cable, or satellite. CSN-Philly does not have them.
What CSN-Philly has, is the Phillys and Sixers. People who arn't MLB or NBA fans don't care, since all college and NFL is available elsewhere.
(yes I know that I didn't mention the Flyers, but the numbers of NHL fans in the US is so low, it has no bearing on viewership)
But the Flyers are big in Philly. In general for the last 5-10 years, the attendance of a Flyers game would exceed that of a Sixers game, and I think ratings were respectable as well. Higher popularity of Flyers to Sixers.
I compare the two as the two play around the same season, with similar number of games, same arena, and given a choice, in the past people (including businesses) would more likely buy Flyers tickets to Sixers tickets at home.