The size of the runway?
Good observation - Robins AFB isn’t far away so AF One could have landed there. We lived in Southern CA before retiring to GA and the presidents would land at LAX and come to OC on Marine One...We lived between two Marine bases (El Toro and Tustin) but Air Force One didn’t go in there.
From 2016:
“Having that extra bit of runway, is just going to make us look all that more attractive to these carriers,” says D’Leon. He says after construction, the runway that is now 6,501 feet will be extended 1,500 feet.””