Because he is the king of disables. His Q does good damage at level 1. Shackle at level 1 holds the target for a whopping 2.5 seconds setting up easy kills, and Serpent Wards push towers down fast. Shadow Shaman usually leads to a lot of first blood situations because of how powerful he is.
Rhasta is a better pusher and disabler then the other. WD and Dazzle suffer tremendously from their poor lane presence, which is sorely missed against the aggressive line-ups we've been seeing in the professional scene.