On a more practical level, where would America be if it supported the overthrow of Spanish rule in Cuba only to be replaced by the English? Or the French? It was no secret that England wanted to install puppet governments in the former colonies of Spain in the New World. These newly minted states were struggling to emerge as independent nations. Most were ruled by a cuadillo – a strong man, military-political leader. The British felt it would be easy to replace any and all of them with their own cuadillos (and they were probably correct in their thinking). There was no doubt in Monroe's mind that American commercial interests would suffer under such an arrangement.
The Monroe Doctrine was a rather hollow threat inasmuch as the United States was not a major power at that time. Fortunately, Britain was focused on war in Europe and didn't entertain any ambitions in the Western Hemisphere at that time. They even went so far as to offer a limited alliance with the United States to enforce the terms of the Monroe Doctrine. However, America was still leery of Britain. The War of 1812 was still fresh in their memories and they suspected any overture from that direction. The exclusion of Cuba and Puerto Rico from the terms of the Monroe Doctrine was a tacit admission that the United States was powerless to force an Old World empire to surrender its existing claims in the New World.