I think I've never raced with the SuperKart 
Well, there's an easy way to fix that.

Maybe we should add it back for the next season...
Maybe.
I think there are several reasons the kart wasn't the biggest hit during its seasons in Zakstunts:
1 The top speed is low but the corner speed and acceleration high. This could give people the feeling that driver skill matters less, a small error costs barely any time.
2 The feel of the car is sometimes a little unintuitive to Stunts players, mostly because of its short wheelbase. The jump length is pretty short, and in the banked corners you sometimes have to steer towards the curve to stay on the road. Cornering is a matter of pressing and releasing the steering keys in any stunts car, but the short wheelbase exaggerates this even further. The top speed is clearly dictated by maximum revs rather than power curve (although this is the case for more cars, particularly those with overdrive bugs).
3 The car may be a little too distinctive. It's good on tracks with lots of corners. Normally every race has several viable cars, so you can pick one you like, but if the kart is getting near the top of its bonus curves and then a curvy track comes up it might become near impossible to compete in something else.
4 The more cartoony looks might not be to everyone's taste either.
I still really like my creation. There isn't a car I've used more since I restarted playing again yesterday. And I think it was cool it got to compete for two seasons. But it has its downsides. Who knows, it might be brought back into the pack one day, but Zakstunts is just as awesome without it. There are good reasons for people to like other cars better.