Maybe they don't want "average viewers" in certain (sub)-regions to stumble upon their familiar presenters sat in front of a different backdrop talking about news from a neighbouring area, thus giving the game away about the pre-recording and resource-sharing that occurs?
Anglia seem to be the only region (?) no longer bothered about this, as they started regularly uploading their east & west 6pm programmes a fair while back.
Disappointed that other regions haven't followed Anglia's lead.
I'm still a European.