Pin 1 remains at top left, as it will be in the diagrams on the datasheet, rather than having to mentally flip everything. That's the possibility I can think of.
When I started wirewrapping, I'd use a magic marker to place a dot at the pin 1 to make it easier to visualize on an upside down board.
For a while I did some wirewrapping for a company (defense contractor) and the boards came drilled and with silkscreened component markings much like you'd see on a PCB. It's not clear why they went through all of the trouble to silkscreen and drill boards for wirewrapping and didn't just make PCB's. Probably a combination of the cost to create PCB's coupled with a low production run (they were building 5 custom products, so a pretty low volume)
