I think it's because Apple mandated tvOS streaming apps had to integrate with the Apple TV app, which a lot of providers did but it meant certain features weren't available that apps used on other platforms. e.g. if you look on the BBC website, I think they say something like Apple didn't support HLG for HDR (but they now do) and subtitles aren't available either because of something or other. Yet ITVX has live subtitles and integrates with the Apple TV app. BBC Sounds isn't available at all...
But I think what I've written above is all probably completely out of date in the fact that iOS supports everything so surely the codebase to get the iOS apps on tvOS cannot be starkly different. BBC is just lazy, they may see the userbase as too small to not bother and I've also read that Apple didn't send the BBC the latest Apple TV boxes so the BBC won't bother fixing their apps for it.
Just decided to nag them a bit via their contact form so we'll see what their latest response is.