I have been playing Mario Kart since 1998. I can get all golds in Mario Kart 64 without hassle; I can place first in every race of Double Dash's All-Cup Tour somewhat easily, I get first for 32 consecutive races effortlessly in Mario Kart DS (without snaking), I have a VS rating in the 8000's in Mario Kart Wii.
I cannot get more than a few one-star ratings in Mario Kart Super Circuit (most of them being in the B-D range), and after getting Super Mario Kart on the Virtual Console last week, I still can't finish the 100cc Flower Cup.
So, what do both 2D games have in common that appears to put them in a separate class of difficulty than all the 3D ones? There are a few things like the coin mechanic and the controls that seem likely, but it doesn't seem like they're the only culprit.