Probably because the plates with the numbers on and fitting them to the train cost rather more than just sticking the numbers on under the windscreens?
When the Selhurst units started getting the same treatment a while later on, it was very short lived and only a small number of units were thus treated; they also fairly quickly reverted to under the windscreens.
That area above the destination blinds is a combination of several panels, the window surround, the destination window surround and the roof panel, all fixed together. Any additional platework (just to carry a number) would have had to be fixed atop the rest, providing an ideal source for moisture ingress and corrosion.
Back in BR (possibly into SWT days, although my memory is now vague on the dates) some 455's suffered water ingress at the point above the cab rear bulkhead, ie above and behind the drivers head, where the main Mk3 style roof panel was attached to the cab roof panel.
This became another job that one particular chap became involved in at Wimbledon as I recall, who spent most of his time on top of 455 cabs rectifying same.