No single reason why only that one. I don't think that I was as aware of the chipping on those other routes at the time.
The chopped route was very new at the time and was quite an eyesore, especially as a good-sized flake had broken off and been concreted back onto the wall - you can still see the vestiges of the concrete. The flake came off again with a light tap of the hammer. There's a fragment of it in my garden.
Anyway there's only so much righteous indignation to be drawn on.
Oh, and the big scar down the bottom of the route pre-dates all this nonsense - probably from the original uncompleted aid bolt ladder back in the early seventies.