"I guess it's because they have had that copy in
the video library since they got it in 1973, and it
would be expensive to replace."

Heck, they might have received it last week. HD tape stock still isn't that cheap, and many broadcasters and content distributors have a lot of infrastructure and supply devoted to Digital Betacam. Independent broadcasters and small-market affiliates may not be hauling in that much money via advertising or, in the US, carriage fees, so it's probably cheaper to order the SD version. Even pay specialty channels will do this to save some money for more programming, maintenance, and upgrades, etc. It will take time for a lot of the old stuff to clear out of the system; until then, this will be a problem.

They could buy a decent piece of format converting gear (we have one at work), but again... money :)