They are recent immigrants to the United States from Portugal.
To answer the OP, Most of the early migrants (mostly from Andalucia) who arrived in Latin America spoke in Castillian language and later migrants like Basques, Catalans, Asturians, and Galicians adopted the language of the majority Andalucian settlers in Latin America. In addition, only Seville and for the lesser extent, Cadiz was opened to Latin American trade.
This. There were several basque in the first years, mostly sailors (but also some famous captains and conquistadors), but they where few and got assimilated to the mainstream Spanish speaking society.
Remember also most of the Spanish who came where men: this was true for all immigrants, but especially for Basque sailors, and isolated individuals rarely pass their language to their descendants.Basques and Galician families began arriving later, when Castialian was firmly settled as the local language.
Incidentally, maybe it's an archaism, but here in Argentina we still call the language we speak "Castellano" (castilian), which is a more accurate term than "Spanish".
Anyway the way to change this would be first, to have Spain open the Nosthern ports to trade from the Americas, and to allow immigrants to the Americas to go throush these ports. I'm sure you'll get way more galicians and Basques in the first years. Maybe if there where fewer people intrested in going, the Spanish Crown would be less restrictive vis a vis who could go or from which port should they go.
This might not be enough, though. You might need to change the pattern of Spain colonization, and to have more families (or even communities of some sort) sent to the Americas insted of isoleted and selected men, who where mostly "old Christians", and predominantly Andalucians (since the only ports which could trade with the Americas where). Spain began encouraging peasant families to go only in the late XVIII, IIRC.