The premise of this question seems a bit flawed.

The reason that Spanish was such a common language in the Americas is due to the fact that the Spanish had a tangible and important presence in those regions. For example, the Aztecs and other indigenous peoples of what is today Mexico were part of the Viceroyalty of New Spain (Virreynato de Nueva España), meaning that they had practical reasons to know the Spanish language (in this case, Castillian). While the Spanish and Portuguese made plenty of formal claims on the lands and peoples of the Americas there is a huge difference between saying "this vast stretch of land is mine" and having an actual presence in those regions, much less effectively governing them.

By and large Spain had at best a spotty presence North of modern day Mexico, such as in what are today Texas and California, but weren´t really present on the Atlantic seaboard of North America. Florida is the territory which had the heaviest Spanish presence of the entire Atlantic coast and by the early 19th century (when it was sold to the US under duress) the settler population was extremely scarce compared to other Spanish territories.

The question then becomes, why didn´t the Spanish settle the North American Atlantic coast?

This is a really complicated question which I´m only partially qualified to answer as it involves Spanish settler policies during the 16th and 17th centuries, but I can at least explain the essential issues at play.

When Spain settled the Americas in the 16th century it was, initially, not entirely clear what kind of colonization they would undertake. One of the key characteristics of the colonization became that colonists sought to build themselves up as a sort of aristocracy ruling over local populations. Conquistadors, fortune seekers, etc., came to the Americas seeking not to reproduce Spanish small-holdings but rather to strike it rich and be able to return to Spain (or stay in America) having ascended several rungs in the existing social order. The Spanish crown also saw colonization in terms of the extraction of precious metals and subjugation of centers of indigenous population and civilization.

After the conquest of the Aztecs and Incas and as more mineral wealth was discovered, the Spanish colonial empire in the Americas evolved around the systematic extraction of this wealth and the attempt to monopolize commerce with the colonies (both through the fleet system). The territories which are now México, Perú, and Bolívia were the valuable possessions which financed the Spanish empire and its immense costs and excesses. Other territories, like Cuba (a major harbor and strategic point for the fleet) and Argentina (Río de la Plata, which transported Spanish silver, hence Silver River), were articulated around that.

Other territories saw their growth tied to the socio-economic transformations in many European colonies in the Americas surrounding the rise of the plantation system. Venezuela´s importance by independence (early 19th century) was due to to the value of its cacao producing plantations. Cuba´s economy transformed along similar lines. Spain under the new Bourbon monarchy (who came into power in the early 18th century) tried to reform their colonial system to incorporate these increasingly important sources of wealth.

All of this left out Atlantic America. There was no obvious indigenous civilization with an extremely dense population and immense riches calling to avaricious conquistadors like a siren´s call. It was too far North for extensive plantation economies. New Orleans was a part of Louisiana and was perhaps the partial exception to this, but its colonial population was almost entirely limited to New Orleans, and for much of this period it was either uncolonized or a possession of France. Fur trappers also hunted up and down the Mississippi but this was more typical of French and English colonization than that of Spain.

Perhaps given time the Spanish would have colonized the Atlantic coast of North America but they had other priorities. Their population also had wills of their own and preferred going after centers of immense wealth instead of doing the hard (and potentially dangerous) work of trying to colonize North America. Spain was barely able to protect its already existing colonies from depredations and was badly overextended by the time England and France began seriously colonizing Atlantic North America. The numerous successful piratical campaigns of the 16th and 17th century against soft Spanish targets proved this time and again. It would have been, practically speaking, imprudent to try to grasp for more territory, especially when they could barely defend what they had and new lands offered little promise of gold or silver.

The following bibliography are pretty good places to start.

Bibliography:

John Elliot: Empires of the Atlantic World: Britain and Spain in America 1492-1830.

John Elliot: Imperial Spain: 1469-1716.

Henry Kamen: Empire: How Spain Became a World Power, 1492-1763.

Tulio Halperin Donghi: The Contemporary History of Latin America.