In Burning Crusade, groups of 4 elemental shaman were proven to excel at arena PvP (with a 2nd person healing as the 5th character). 4-5 shaman were also proven to excel as 5 straight-dps-with-earthbind-and-frostshock-kiting heroics or with a tank as a more standard healer/dps/tank group. They also were able to function well as part of a raid (I think Nixi even had a group of shaman, and Suvega and I were a mix of shaman/mages)

Basically, as a hybrid class with automatic defensive abilities that protect multiboxer weaknesses (grounding/tremor) and mechanics that encouraged stacking (totem combinations), they were able to achieve success in all aspects of the game.

That's why they're popular.



For my (dated) analysis at the start of the "flavor of the month", see Origins of the Shammy+Pally Fad/Phase/Craze/Insanity(?)
(jeez, that was a YEAR ago!)