AV-8B is a fast jet, true, but that works against it in the majority of CAS missions.

The Harrier may be able to fly conventional, but what’s the point? It’s meant to be deployed in a VTOL role. If a runway is availabe then there are much better options than overloading the Harrier. The reason you don’t want to operate Harriers out in the open is it’s obvious vulnerability to small arms fire.

The A-10 has a few advantages over the AV-8B that would lend to it’s higher survival rate. The cockpit armor. The second engine. The design of the wing to remain controllable when half of it is removed or inert. A second vertical stabilizer. The standardized communication gear allowing it to talk to everyone by default, not just other marines. Those are just a number of reasons the A-10 is superior for CAS in my opinion. I’m guessing more thought was put in the A-10’s survival against the SAM threat, too.

The majority of a marine pilot’s flight time involves coming and going from dry land. I’m thinking they don’t need a lot of modification for USMC duty operating from dry land.