I would imagine the racial disparity is really just a result of the socio-economic disparity. As for why that exists, idk. Might be the cost of having a smartphone. As far as I know, Snapchat is pretty worthless without a smartphone. It’s closer to a messaging app than a pure social media service. Facebook on the other hand is usable on a computer, so even teens without smartphones can use it at school or the library or on a shared computer.
It is strange, indeed. I have been reviewing what seems to be the original study: http://www.pewinternet.org/2018/05/31/teens-social-media-tec.... It includes some detailed figures about these disparities, but they don't seem to provide insights nor opinions on which reasons could be leading to those differences.
A wild guess here: assuming that there is a correlation between higher-income and higher-education groups (quite likely), it could be that more educated individuals are more informed about privacy issues, being more wary of their data. The lower Facebook usage find in both groups could be motivated by that (eg recent scandals, like Cambridge Analytica). This guess would explain lower Facebook usage among people with higher education (directly) and also among those with higher income (indirectly due to the assumed correlation with education). That's my two cents, although just a guess.
I remember something similar with MySpace -- Facebook reaped users from the top down (social class-wise), and MySpace seemed to retain the less-privileged.
That’s all conjecture though.