I'd say sp/so is one of the most common variants in general, and there are a TON of IFP sp/sos, saying that they're white is a bit ridiculous. Also sx being the most common for some types is ridiculous, because sx is the most rare.
I think that the way we approach the variants is different and that's all that can be said. I don't think you can really correlate rareness of variants with MBTI type, they just appear to be rare/common in certain cases I imagine because either that particular variant is common in general, or you see far too many of these more "common" ones on the internet and think that they're common because of that. There may be a decent bit of sx on the internet, but in reality it's quite rare.
Sx serves us literally no purpose in a primal sense, I have quite a few theories on the instincts and evolution but I'll spare that for another time. Humans have strength in numbers, we are social animals at heart, and yet aside from that we have a sense of self preservation as well. Sx lasts are the most common type across the board for humankind for sure. I've given a lot of thought to whether sp or soc is more common, and I think that soc is more common (at least at the moment, it just makes sense to me in theory) and there are just soc types everywhere. But then again there are sp types everywhere too, but to a lesser degree.
Sx types being more "Common" is a dream that probably won't come true. If you know a lot of sx types then it's probably because of a particular interest of yours, but in general the bulk of the human population with be soc and sp.