. . . Some say its the economy (less money to blow on entertainment), some say to high a learning curve (new players get frustrated and leave), some say the graphics are out dated (new graphics are in the works, but is it too little to late?), some say its the toxic environment ( major bullying and verbal abuse). I think it's a bit of "all of the above".
Add to that an ever declining PC sales. Tablet sales will exceed PC and laptop sales combined next year.
Keep in mind too cell service has become a major hit on folks budget.
Game systems like Xbox.
All factors in these things reduce player numbers.