Short answer: they are a mediocre team minus a superstar and lacking an identity that's worth paying attention to.
The longer version is slightly more complicated.
Aside from the Big 5 college basketball teams (Villanova, Temple, La Salle, Penn, and St. Joe's), high school basketball has long been a staple of Philadelphia sports discussion with schools like Neumann-Goretti, Gratz, and Chester always near the top of the PI-AA standings. The Public and Catholic Leagues have produced products like Wilt Chamberrlain, Tom Gola, and Rasheed Wallace, and more recently, guys like Syracuse's Scoop Jardine and Rick Jackson, Pitt's Brad Wanamaker, Nova's Maalik Wayns, and the Morris twins at Kansas that have all gotten publicity on a national collegiate level.
With Sonny Hill and John Chaney spearheading the youth basketball movement in the city, the focus is always on the local rather than the professional. And without a team to be proud of in the Association, people are content to pretend like they don't exist, or at least only marginally so.
Also, being one of only 13 cities with four professional sports teams, it's simply difficult for some fans to focus on all of them. Especially when the other three have such a loyal following over the past decade or more.
The Eagles will be tops in this town as long as they stay in the arena of relevancy, just because of how short the football season is. Americans, and in our case Philadelphians, suffer from the mostly undiagnosed disorder of a limited collective attention span. So when you tell them they only have to watch them play for three hours once a week for 16 weeks, they can spare their time. Being over .500 for every season but one since the old millennium doesn't hurt either.
With Philadelphia wearing its reputation as a blue-collar town on its sleeve, hockey has always been a favorite around these parts. People like watching people get hit, and the Broad Street Bullies of the mid-70s still carry enough weight to get folks watching. Also, they've had enough success to keep people around and have been at the top of the NHL this season. Chicks tend to dig hockey more than basketball anyway, so it makes for a better date as well.
But when it comes to combining a good date with success, no team has compared with the Phillies of late. Despite a grueling 162-game schedule, baseball is such a staple of summer that Citizens Bank Park is always filled to capacity (or close to it, depending on who you ask). Being the only sport during those months is a huge boost, though the Vet era of horrific teams year after year didn't exactly show off a packed stadium.
Basically, if a team is good, the fans will show up. When the team isn't is when it gets tricky. The Phillies will always get fans because summer nights in Philly are the perfect times to catch a ballgame. As long as hockey doesn't endure many more holdouts, the Flyers will always draw a loyal crowd. And unless Doug Pederson puts on the pads again, the Eagles will be okay.
The Sixers don't benefit from the loyalty of the Philly fans as much as their three counterparts. With only 69% of the seats at the Wells Fargo Center filling up for Sixers games, Comcast owns the worst percentage in the NBA. Unfortunately for Sixers fans, the ownership hasn't shown that they care enough to commit to anything other than mediocrity and a slight profit. If a superstar falls into their lap and they can market a young core on the rise, you can be sure fans will show up.
They've just got to work harder for them than the other three.