As Alayan said we will never know, but historically the predecessor of SuperTuxKart wast called TuxKart and was already joined. When the team forked it and added the word super they most likely just appended it. Further more there is also a game called SuperTux so naturally the temptation to not add space was high.
Sometimes the most valuable thing an old wizard can offer isn't answers, but the courage to ask: "Why shouldn't we?"