I've lived here for the majority of my life. Philly isn't a big market basketball city. I dare you to name a big name free agent (aside from the aforementioned Elton Brand) that has came here since 1990. During the Iverson era, he was drafted and the majority of that team was acquired via trade.
The general Philly sports fan is a front runner. Even when the Sixers were a 6-8 seed, they were among the lowest 1/3 of teams in attendance. When the new owners bought the team, there were even rumors they wanted to move the team to NJ (since they don't own Wells Fargo, but own the Prudential Center in NJ).
To the first point, the fact that free agents aren't clamouring to join Philly is less of an indication of their market and more of an indication of their management and how free agents view rebuilding. In spite of New York's big market appeal, we also failed to bring in any elite free agents over the latter few seasons simply because we didn't look like a team that could win. The Spurs, Heat, Bucks, among others, all managed to bring in free agents and they're not big market basketball cities. Philly is easily among the top 10 biggest market basketball cities in the US, their history didn't start in the 90s.
Their current moneyball philosophy (which I would argue is actually not that) whereby they focus solely on youth is a blueprint that has not been replicated elsewhere, it just sounds like a great philosophy. Teams like the Thunder lucked into the lottery picks that resulted in Durant and Westbrook, and then very quickly built a team and philosophy around them, others like Houston wisely manouvered enough cap space at just the right time in the hope they would luck into some free agents and Harden happened to fall onto their laps. Neither of them went through several seasons where they won less than 20 games. You can't plan a philosophy around lottery picks and end up succeeding and that's the issue, it's not a guarantee. The Sixers have had the worst run (record wise) over the latter few seasons than any other team (including the expansion teams when they started out), and that isn't about their market size. And that is why free agents have , and will continue to, avoid them.
To the latter highlighted point, there are very few fanbases that support their team irregardless of their record. Very few. For the most part, either a winning season , or a superstar player attract the fans. Since Iverson, has there been any player that could have made it to the All NBA 3rd team? Iguodala is your best shout, but even he wasn't considered among the best in his position. Elton Brand might have become one if he hadn't succumbed to injuries. And 6th place in the East isn't exactly a highlight, since the 2000 season the Sixers have only had 4 seasons when they've won more than 40 games, and only
one season when they had more than 45 wins(and that was 48 wins). That's over 15 seasons, in the East! That will challenge any fanbase.
The Philly fans deserve better, anyone over 30-35+ can remember when they were a bonafide force in the league, now they're clutching at the hope of a lottery, followed by the hope that potential will turn to reality, followed by the hope that that results in them becoming a contender. In the meantime they're watching their team lose at a rate never seen before.