I am possibly the only berk here who has worked in television back in the 60s, so with that in mind may I point to Hugh's statements here -
Hugh Robjohns wrote:I think there a lots of factors in this... The terrible sound quality of the built-in (and usually rear-facing) speakers of most flat-screen tellies certainly doesn't help.
And neither does the fact that most young TV directors seem to think they're aspiring Hollywood directors and approach their film-making with the wrong mindset completely.
And its made worse by the very nature of independent programme making, which often means post-production in film-style dubbing theatres with big screens and surround sound.
And then there's the training... Back in my day, the BBC effectively trained the entire UK industry. It doesn't anymore and hasn't for a long time. It's entirely reliant on universities and specialist colleges, and while a couple are good, I perceive a bias in expectations towards the film industry rather than TV.
Those are the reasons all gathered together in a nutshell (but missing a couple of important additional reasons).
1. Back in the 60 and through to the 90s, we were working through a fog of poor equipment (compared to modern digital) and very constrained budgets. The studio I worked at in the 60s had three cameras which had to do everything for two large and very busy studios. You had two camera shots, medium and close-up, as anything wde would leave viewers confused as to what was going on. With just 405 lines of black-and-white, you can't have loads of wide shots to tell a story. That meant that mics could be close to the actor and actors had to speak clearly.
2. Stories in dramas were driven by dialog, so words were important. Early TV was really just radio with pictures - you could turn the image off and still follow the story. Everything was verbal. Visual gags and cues were few and far between. Again, that meant clear enunciation of words - no mumbling!
3. Actors came to television from the stage and that meant declaring every word. The struggle to fight through the fog of 405 lines and poor audio was very similar to projecting to the gods in a theatre. Also EVERY actor had to study Shakespeare and the ham-acting of over-declaration of texts was very much 'en vogue'. Every Charlie on the boards thought they were Sir Laurence Olivier on the boards at the Old Vic. Today, they all think they must mumble like Dustin Hoffman!
4. The Sennheiser MKH416 only became widely adopted in the 80s and the latest version of that mic (now the absolute soundstage standard) is simply stunningly clear - so much so, that I am now often using it for music recording. Until then, we had all sorts of mics, none of which were all that good! Actors had to speak-up!
5. Modern speech is slurred. Even junior royals and the prime minister speak with semi-working-class accents that swallow syllables. This is just the way people pronounce their words and we have to live with it! A very stilted and over-clear pronunciation developed during the Victorian age and this evolved into 'The King's English' or 'Received Pronunciation' and more commonly 'BBC English' and unless you listen to the Queen or some member of the so-called upper classes of that vintage, you will not hear that anywhere.
6. Modern scripts are often poorly written by people with an incomplete command of the English language. A modern script is very likely to have grammatical mistakes and typical slurred idioms already written into it. For example, Maggie Smith's character in Downton Abby spoke the words "Then I must have said it wrong!" That is both a massive grammatical mistake and a Cockney idiom and certainly not the language of a 'dowager countess from the Edwardian era! (There were similar howlers in the BBC's 3rd-rate adaptation of 'War of the Worlds' last Sunday!)
7. As Hugh so rightly points out - every director wants to be Sam Mendes and every cameraman thinks they are Roger Deakins. (BTW - watch out for Mendes' and Deakins' new movie '1917' coming in December - I've seen some of the 'rushes' and if it lives up to that which I have seen so far, it is going to change the way movies are made and what we expect from an action movie!)
8. Add to that, broadcasters are viewing the final product on either very large 4K monitors and with 5.1 sound systems, or even in a proper viewing theatre - hardly the pokey little set-up most people have in the UK TV market in their living rooms and bedrooms!
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Back in the 80s when I was a the forefront of broadcasting, When I finished editing a programme it had a 'tech review' which meant watching it critically -- usually with the director sat beside me -- on a 'Grade 1' CRT television (625 lines -- I'm not as old as Andrew! 
) while listening on a 'Grade 1' speaker (usually an LS5/8) in a room with decent acoustics and lighting. Top quality for the day... 
