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.
PHL is finally getting one this year. Frankly I'd kill for CLT to have an Admirals Club for each pier ala DFW before even thinking about Flagship Lounges