To a large degree, the answer goes back to Slavery in the Americas. Slavery has existed since the earliest recorded times. But as the American slave trade started, and huge numbers of Africans were shipped to the west Indies, and eventually the American colonies, people began to associate the institution with race. Also, since slavery was not a large part of European culture(they had serfdom early on, and after the black plague it was an economially unsound way of doing business, though it was not actually illegal in many european nations), people did not see anyone other than blacks enslaved.
As the age of Reason dawned, and people began to question the morality of slavery, those invested in the economics of slavery had to find reasons to justify their position. Religious ones were the most common reason, and the two passages you quoted were the ones they believed best justified slavery. And yes, it is a lot more than a stretch to say that the mark of Cain was to make him black(given the mark actually afforded him protection from harm), or that Canaan and his decendants were suddenly black.
Other reasons given were equally unconvincing. The most comon was "it's for their own good"
. Whether because they were converted to Christianity, and thus had their souls saved, or because they were "provided" regular food and clothing, or because they were taught "civilized" languages or technology or culture, those who believed in slavery found it easier to come up with justifications than face the fact of what they gained from the institution. Either they benefited economically by owning a slave or slaves, or they gained socially, because no matter how poor or uneducated a white man was, he was infinitely higher in standing socially than the most educated of slaves.