^^^ Cynical it may be but it's almost certainly correct.
Quote:
"It is also important to note that not all programmes will be available on BBC iPlayer due to rights restrictions"
Source:
https://www.bbc.co.uk/iplayer/help/programme-availability/programme-availability-info/programmes_after_broadcast
Studio Canal hold the distribution rights to the series, which (as far as broadcasting is concerned) they've sold to Amazon Prime, rather than to the BBC. They're also selling the DVD themselves (£12.99 from Amazon), so the BBC isn't able to retain the series on iPlayer.
As streaming services, such as Amazon Prime and Netflix, continue to buy up the broadcasting rights to series which have been premiered on the BBC or ITV, there are likely to be fewer series available without payment on iPlayer or ITV Hub. The BBC and ITV are fighting back with their own streaming service, BritBox, which is about to be launched soon. Just like Amazon Prime and Netflix, it will play host both to series which have already been aired elsewhere and to original series, especially commissioned for Britbox, too. However, just like existing streaming services, you'll have to pay a monthly subscription (£5.99) to access it.