There is a very fascinating book by Alan Strathern called Unearthly Powers which argues religions can be divided into immanentist and transcendentalist. Each category takes a difference in approach to religion, with immanentist religions (as Strathern calls them, "the religions with no name") focused much more on religion and directly immanent to humans and the world around, tended towards emphasizing a nature-involved deities and spirit-filled landscapes. Many of the, as you put it "native beliefs," fall under this category. I am simplifying of course, you should really read at least an article or listen to a lecture by him if you cannot read Unearthly Powers yourself and you are interested in more. The transcendentalist religions are built overtop of immanentist impulses, and are not entirely free of them, leading to an interesting tension, but fundamentally have different core tendencies. With immanentist religions emphasizing the broad spiritual landscape with a need for the deities to provide direct intercession, incoming transcendentalist religions with a supposedly all-powerful deity (evidence of which can be 'proved' immanently, through the believers' greater wealth/technology/connections, etc.) found it easy to convert people. However, when two transcendental religions meet, the religious advantages to conversion vanish. One religions claim to supreme power no longer seems unique, and (if it exists) the other religion's greater materiality doesn't mean your religion is less powerful. You believe a transcendental religion because you believe it. Therefore, the tree of transcendental belief is harder to uproot by proselytizing.

There are other theories of course, and this isn't a historical overview on the various instances of conversion or non-conversion. I'm not even sure I directly hit all you wanted talked about, but it is a very fascinating typology which might shed some light both on this question and on the natural follow-up questions.