I answered a question very much like this one in a thread a few years ago.

Long story short, the coronation of ras Tafari Makkonen as the emperor of Ethiopia happened pretty soon after Marcus Garvey made statements about a messiah for black people arising out of Africa to redeem those in the diaspora. The coronation of a new emperor appeared to fulfill this prophecy, and Ethiopa's place as the only country in Africa not to be a European colony* at that point (1930) was powerful symbolically to a movement that sought the redemption and liberation of black bodies from the legacy of slavery and continued racism.

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*I'm aware of the situation of Liberia as an independent country in 1930. I don't know offhand what Rastafarian thought was regarding Liberia at the time.