I think you can make a case that the reduced legal status of slaves in English vs. Spanish colonies was an unfortunate side effect of the generally more egalitarian English society.
With very rare exceptions, almost none of which were applicable in the colonies, all English freemen were equal under the law. (Actual practice, as in all societies, was somewhat different.)
In Spanish colonies, OTOH, there were massive differences in the legal status of various groups. Notably, if I remember correctly, government officials, military officers and clergy all had their own separate legal systems, not being subject to the general judicial system. In practice this meant these groups were essentially above the law, as “their own kind” was highly resistant to punishing them.
In English colonies you thus had the majority of society in one group. While it hadn’t been verbalized yet, the basic fact was that “all men are created equal.” The only groups outside this were the black slaves and the white convicts and indentured servants. So you have a big group that are “people,” and a smaller group far below them separated by an impassable gulf that are “not people.”
In Spanish colonies nobody was equal. There was a spectrum of status and rights from the slave to the Viceroy, with very slight differences between any two adjacent individual statuses (statii?).
So the Spanish had no difficulty thinking of slaves as people, they were just people with a different and lower status.
The English and later Americans had considerable difficulty doing so. All people are equal, but the slaves aren’t equal, therefore they must not really be people in some sense. This notion reached its highest (lowest?) point with the Dred Scott decision.
Interestingly, the English colonies in the New World dropped common law when it came to slaves and imported the foreign Roman civil law definition of slaves as “speaking animals,” with no rights whatsoever under the law. This was softened to varying degrees in different areas by the influence of Christianity.
Did English common law even recognize slavery as a status in the 16th century? By then the English are involved in the slave trade, but perhaps not taking any of those captured to England. The Spanish and Portuguese had a tradition going back to Roman law about slavery, reaffirmed by the church in the 1400s...one of the legitimate categories justifying slavery was “taken prisoner in a just war”—so any slaves purchased from dealers at trading posts on the African coast could be classified as prisoners of war taken in a just war. Somehow all the wars that resulted in captives were just wars.