Every time I meet or hear/read about someone named Marcus, it’s almost always a black guy. I have known one white Marcus in my life; his father was German. I gather that Marcus is a German name. It seems to be exceptionally rare among white people in America.
Roman names had a long popularity in black communities after slavery. Cassius Clay was a junior, for example, and his father was named after a white abolitionist. Rome was a symbol of republicanism in the early America so lots of Roman names existed in the white community before the Civil War. When blacks started being able to name themselves, it seems likely that they chose names popular or meaningful for the period.
I know this is what all the baby name books say, but it is not correct. Mars, gen. Martis, looks nothing at all like Marcus, gen. Marci, other than the same first syllable. True, -c- and -t- get confused in Medieval Latin, but not in Classical Latin. Plus Old Latin is Mamors, even less like Marcus. As far as I am aware, the actual etymology of Marcus is not known.
And just to add, I know a couple of white Marcuses, one of whom is under 10 and the others of whom are in their 40s.
Also a lot of African American males do not like to shorten their names. I know a lot of African Americans named James, not Jim; named Terrence, not Terry; Andrew, not Andy and so on.
Mark, Marc, Marcus, don’t seem to be that common names. I have only known one or two named that or their variants. So when you hear Marcus, it may seem more common among African Americans as others may shorten it to Marc.
Yes, Marcus is common, but I don’t know if there is a solid reason. Why is Aiden so trendy now among white folk? Because people here the name and like it. I don’t think it’s anything more complicated than that.
They’re only as good as their sources, and given names are notoriously difficult with regard to etymologies. The Marcus < Mars works as long as you don’t look too closely. There’s a plethora of data, including the name Mamercus, which does seem to be from the god. As you say, irrelevant to the discussion here, anyway.