RDU is a major airport along one of the busiest air traffic corridors in the world. There are multiple SIDs and STARs to coordinate getting traffic into the flow at altitude along the East Coast. Between NY Center, Washington Center, and Atlanta Center, things get pretty congested along that area, especially with DCA, CLT, PHL, and CLT arrivals all being in the vicinity.
It isn't about the size of the airport, it is about getting the traffic from that airport into the enroute structure in an organized way and with multiple options depending on congestion.
Plus, RDU is a major diversion airport on the East Coast for just about every carrier. I have seen 25+ AA airplanes stacked up there when CLT closes due to TS in the area.