I'm not asking for evidence, just a reasonable argument as to why people might learn better at night. Because the obvious thinking on that is, until very recently, night time meant most humans simply went to bed.
I would say that variability in ability to concentrate might be reasonable to assume as a starting point, given how much people vary in other features as well, even if there are similar connections to how humans evolved. For example, food preferences are also in part a result of evolutionary processes, but vary a lot between people.