Why the RP accent is mainly used for international audiences

In this interview American's struggle to understand colloquial, non-RP accents is demonstrated.
He says people not knowing what he's saying is "his world".

In Money Monster, when playing grief-stricken antagonist character, he could have played it with his normal accent, it would even have gone with the British villain stereotype. But '71 proved that a protagonist with both a working class background and non RP-accent does not sell well, as well as his character not having enough of the conventional hero archetypes (no love story, not enough confidence in personality, no real interaction with other characters, has no real power in the narrative, is nearly killed in the end and is the one who has to be saved). This makes the film less marketable to a mainstream audience for a wide release.