BA left as the B733s and B735s were withdrawn from what had been a CityFlyer Express operation on ATRs and RJ100s. They no longer flew an aircraft type best suited to the route.
Also the hub at LGW was closed so the route became wholly point to point after the first wave as all remaining long haul had departed, as you said, the remaining aircraft were deployed to routes elsewhere, where Avios holders could burn miles and therefore stay loyal to more profitable BA routes elsewhere.