This happens every year. He might just not be sure he wants to play next year, or maybe it's just that after the initial rush ends, teams realize they aren't getting their ideal guy, and players realize they aren't getting their ideal pay day, they take more time and consider fit, location, etc. A couple dozen veterans who are still useful take training camp invites or low money deals in September every year. Also, Stempniak is 35 and had nine points last year, so like, maybe just nobody wants him?
It's not weird that a potentially useful veteran is still available a whole three weeks into free agency. It's weird that teams rush out to give multi-year contracts to Jay Beagle and Antoine Roussell on day one.