This is the exact question addressed by Marshall Sahlins in a number of books and articles, such as 'the stranger king' from 2008 but the idea is older. He (and other recent scholars) have pointed out that rather than some 'winning' outcome of the 'competition between westerners and natives (the path that Jared Diamond takes), it is in fact a part of native ideologies to value the strange and the foreign in electing authorities precisely because they are foreign and thus enable a renegotiation of local power interactions. This is also ingrained in native ideologies, with kings and others acting out creation stories (which also feature foreign origins) in cosmogenetic displays. In my opinion it is extremely difficult to find single causes for this widespread phenomenon of stranger kingship in so many varied societies as South-East Asia, Oceania, the Carribean and even Old Norse society, but the existence of such integration of the foreign in local power structures has been recognised for some time, including in Mary Helms book Ulysses' Sail.