Game Freak wanted it to be like it. I'll edit this if I find something.
>It has a Shellder latched to its tail in a symbiotic, almost parasitic relationship. The extra weight on its tail has caused the Slowpoke to become bipedal. As a result of carrying this burden, its special abilities have developed greatly. Through the symbiotic relationship, the latched Shellder grows in a spiral shell instead of a bivalve.
Shellder changed it's shape due to having a mutual relationship with Slowbro.