Maybe it's because it's Kraid's size. It wouldn't make sense for him to be able to keep up with Samus story-wise. The only major enemies that I can remember that could keep up with Samus were Ridley, Dark Samus, and SA-X. The odds on running into Kraid on another planet are slim but he is due for another battle with Samus. maybe he has even lost some wait or something, only time will tell of his dreaded fate.