My theory is that the group was so large that it formed its own society and as Quebec is so close, there was a constant and continuing connection with the old country...sort of like with the Mexicans. The Irish, Greeks, Poles, Jews etc didn't have that. I can say, however, that for recent immigrants from just about anywhere avoiding assimilation and staying connected to the old country is a snap with satellite TV and the internet.
And you can add to your theory the fact that the government of Mexico encourages illegal migration to the US as a major part of its economic policy. Those in the US are still considered Mexican, and are encouraged to maintain ties with Mexico, to behave as dual citizens.
Mexico wants it excess population to enter the US illegally, to get rid of it and avoid a revolt among those who found no real opportunity in Mexico.