I think the problem is that we've currently got a) pressure to give more roles to non-white actors, b) a risk-averse culture in entertainment where there is less original writing than there was and everything has to be a rewrite or regurgitation or franchise of something we have already. And so you get yet ANOTHER Christie adaptation/Henry VIII saga/Austen drama, and then the diversity box gets ticked by crayoning in some of the characters as being non-white.
Thing is, the UK was genuinely an overwhelmingly white country until quite recently, and a visibly non-white person walking around in the 1950s UK village would not be going around without a lot of comments, stares and noticeable...."reactions."
So you've then got this awkward dilemma of, OK, so do you actually reflect that reality? If you do reflect that reality and have the white characters reacting realistically to non-white people as they would have done, the whole drama is likely to end up being "about" race. That's not an issue in itself, but when more and more dramas do this, it does start to feel tiresomely samey and even people who are not unsympathetic to the politics themselves are, I think, starting to get a bit twitchy and feel like "I'm basically getting a sociological lecture every time I watch drama on TV." Or, if you choose to have everyone "act colorblind," with all the white characters just treating the non-white characters in a neutral and unexceptional way and apparently not even noticing race, you are essentially lying about the past in a pretty major way. No, I don't expect historical dramas to be 100% accurate fascimiles of the past complete with smallpox marks, but wholescale untruths about the values and attitudes of people make historical dramas feel pretty fake, TBH.
Murder is Easy is actually not very good Christie, but this adaptation doesn't improve it. The real issue of MIE is the silly main premise of the murders themselves (mad and incredibly elaborate conspiracy theory - sorry, trying not to create spoilers for those who have not read or watched) - rewriting that bit would improve the story.
If we want more dramas with non-white characters in, well, a complete rewrite of MIE set in modern times (a la "Clueless" as a modern take on Emma) would be interesting and you could put what kinds and colors of people you like in it. Or, just write some new and original material!