There's only limited capacity available on the transmitters that broadcast Freeview signals, meaning that some rather strange compromises are needed. For example, Talking Pictures TV broadcasts in standard definition but there's only space for it on a high definition 'multiplex' (= 'bundle of channels'), so it's only available to viewers who've got 'Freeview HD' sets (rather than just ordinary 'Freeview' ones), even though it can't be watched in high definition anyway. (There are a few other channels in a similar position).
Similarly, when the BBC wanted to expand their HD services on Freeview, something else 'had to give', and that was the number of streams that could be carried by their Red Button services. So viewers with basic Freeview sets are now limited to just 3 streams (while those watching via satellite can get 8). It's been annoying people ever since the London Olympics:
https://conversation.which.co.uk/technology/olympic-tv-bbc-freeview-coverage-red-button-sky/
I believe (but I'm not 100% certain) that the BBC has now 'solved' the problem by routing Red Button services over the internet. So Freeview viewers who've got the most recent models of 'smart' TVs can now get full coverage of Red Button services. However I suspect that, like me, you're stuck with an older telly, which can't provide the full range of services.