The early 1600s was a tumultuous time in Europe. Religious wars tore through the continent causing many to flee to other parts of the globe as refugees. No Europeans were affected more strongly in this than the non-human species who hid themselves within the various human populations. What made matters especially bad for them was the English independence from the Catholic Church.
The Church reacted to this apostasy by persecuting any other groups they saw as a threat to the faith of their followers. Non-human species of various kinds found themselves hounded by agents of the church. Holy warriors and priests sought to cleanse the land of evil, and they didn’t much care if who they cleansed were actually threats. Many innocents died in the conflicts and many more found the level of harassment unbearable.
Meanwhile, in the Protestant lands, the strength of the newly emerging English system for magic led to a resurgence in the dominance of humans. While non-humans were not as actively persecuted, they did find their roles in society diminished. This effect slowly spread throughout Protestant Europe as other nations followed the English example to organize their magic users into a unified school of magic.
So, as a flood of colonists headed towards the New World in search of a better life, their numbers were disproportionately made up of non-humans. The human colonists were still in the majority, but many creatures who had only a minor population to start with found themselves present in significant numbers. For some species, it was the first time they had ever achieved a local majority even if in the grand scheme of the Americas they were still a minority.
Centuries later in the late 1700s, this process was still in place. However, by this time many of these non-human species had well established themselves and held prominent places in colonial society. When the British Thirteen Colonies moved for succession, a significant portion of the influential individuals involved in the process were not human.
As some examples, records indicate that Thomas Jefferson was a Sidhe, Benjamin Franklin was an Incubus, Alexander Hamilton was a Selkie, and George Washington was most likely a Werewolf (though reports conflict and if this is true he likely had a superb level of control over the transformation). This kind of early influence of non-humans in the political process meant that as the nation moved forward fully incorporating non-human monsters into their society became a norm for the American system.
In modern times, humans maintain a plurality of the population at 45%, though they lost the majority long ago. This has significant implications for the American military. Where most militaries of the planet employ primarily humans with a few non-humans in specialists roles, the American military is predominantly non-human. Humans do have their places, and many don’t even realize that their comrades are not the same species, but humans make up a minority of those in uniform.
This is most extreme with special forces units, which often get as low as 10% human, and what humans do remain present often have specialized magical talents. It is rare for a human without the magical talent to qualify for such elite units off of pure physical ability. No other nation deploys as many non-human military personnel by raw numbers. There are many smaller nations that are larger in percentage but small in overall size. Romania features non-human combatants in prominent roles as a standard practice, but keeps them to elite positions and makes up the bulk of their forces with humans.