Some interesting speculations here.

First of all, I believe that much Spanish immigration into the Americas was from Galicia and other northern parts of Iberia. At least, the sleeve notes to the Chieftains' "Santiago" album seem to suggest as much. This may explain why the Moorish influence seems to be less over there.

Secondly, there are of course regions where the modes which are rare in the English or Celtic derived traditions are much more common. The Lydian mode is common in Eastern Europe, and there is much Phrygian-sounding music in flamenco which suggests that the Phrygian should be more common in the middle east which I believe has influenced flamenco.