Portuguese and Castillian both had massive arabic influences in pronunciation and vocabulary, not only because of demographics, but also because the Muslims had a more advanced and refined culture at the time. Still,

- they both remained mostly latin languages, and

- that influence had already happened when American colonization began. In Spain, the Muslim and Jewish were banned the very year America was discovered.

So the only explanatory power of that fact may be that

- Portuguese colonies were repopulated mainly from the north of the country (as was the case at least for Brazil), and

- that the southern dialects of the language have been promoted as the standard and expanded throughout the country. This is true today, where mass media are washing away the northern phonetics (actually richer: they distinguish between x and ch and -om and -am), accents and vocabulary in new generations.