I guess they need a bit of work to make them airworthy and suitable for the civil register so too costly to fly them all out.
Despite being low houred they are the only opetational Tristars in the world which I can only imagine makes their operating costs high. Likely a big factor the RAF didnt keep them any longer.
My guess is the ones with the big cargo doors will be the flyers. They are the lowest hours frames too I think.