It's a matter of history, the former USAir hubs don't have flagship lounge while the legacy AA hubs do. They also have to compete with other carriers at ORD, while at CLT they have basically no competition.
There are plans for a flagship lounge at CLT, but it requires a lot of construction.
AA doesn't have Flagship lounges at the ex US hubs but US Airways did have the Envoy lounge there which was quite nice.