Because it's not!
Actually, I am not happy with Ray's description of any accent as 'relatively unmarked'. As there is no neutral accent, then all accents must be marked. The difference between the Northern Englishman's /bʊt/and my /bʌt/ is not that his is marked and mine not (or vice versa). They are both marked.
Until my early 20s, I always considered Southern British RP to be unmarked (no relatively about it) - it was what my grammar school teachers used, and I heard it on the BBC every day. I took it to be classless and regionless. As I learnt more about that language, I realised it was neither of these things, It was just a dialect, albeit one enjoying high prestige.
My friends from the north considered my accent to be heavily marked - for coldness, hypocrsisy, snobbishnes, elitism etc. Some of them seemed to genuinely believe that I put this accent on. They could not imagine how it was possible for someone to speak in that grotesque way naturally.
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