At the beginning, Shellder's shell is only made of a single, smooth layer, so, why, when Shellder is attached to Slowbro, it is spiraled?
This is because the shell is based on a kind of marine snail sometimes known as a sazae. Slowbro and Slowking are based on the sazae-oni, a creature from Japanese folklore known for having a sazae shell attached to its head or lower body. You can read more about it here: http://bulbanews.bulbagarden.net/wiki/On_the_Origin_of_Species:_Slowpoke,_Slowbro_and_Slowking
The most probable reason that there could be, would be that when a Shellder clamps on to the tail of a Slowpoke, they BOTH evolve to form one Pokemon. This evolution in probably different and unique and so the the Shellder isn't a separate Pokemon any longer and so is a part of the Slowbro. It wouldn't have made sense if Slowpoke evolved with a big Cloyster latched onto it.
In other words: GameFreak logic