Another issue, which we tend to forget about in this age of 4K and so on, was that a lot of these shows were filmed on digital betamax or similar (and before that on analog magnetic formats). For a British show, that would typically mean 576i 50Hz PAL, and it would just not look great on most equipment when transmitted as 480i 60Hz NTSC. Some shows were filmed on film (I think this was more common in the US) and those were easier to deal with, but horrible picture quality would have been a concern.