" I don't know why in Simbabwe in particular"

As I understand it, the BBC has already checked with every other overseas network that received copies of the original tapes, but due to political issues, the Zimbabwe government was - perhaps unsurprisingly - unwilling to let the BBC trawl through their entire government media archives... So if original tapes do still exist, Zimbabwe's basically the only place that hasn't been properly searched.

(Though, sadly, even if by some miracle the Zimbabwe Broadcasting Corporation did keep its archives from when it was the Rhodesian state broadcaster, there are plenty of lost episodes that never aired outside the UK, and so are almost certainly gone for good...)