I think it's more to do with that TWOK's original camera negative was in terrible shape when they went back to remaster the films. Unlike with the rest of the movies, they couldn't wait another ten or so years to eventually give it a new transfer, so they did it then, and then yet again for the director's cut before the negative is too far gone.
It's nothing to do with sales, if that was the case, companies like the Criterion Collection who are known are remastering smaller, older films with intense care and passion wouldn't exist. People will buy the Blu-rays, it's just that Paramount thinks the current masters are satisfactory.