Just based on a one day snapshot the market for WoW gold is just so much larger than all other games combined. Those wishing to hide any laundering activity could use the secondary WoW market to launder money without leaving much of a trace. I should add that the size of the market is very volatile, as the amount of ISK for sale at the site jumped from 1.8 trillion ISK to 10.9 trillion ISK in the course of 24 hours this weekend.