The worlds are nearly empty for four reasons, in my (not-so) humble opinion:
1) Conte's Inferno (the initial release of 2.0) was so bad - and the way WIS defended it as "working as intended" or worse, our fault that we didn't understand the nuances of the new engine - drove away many.
2) They reduced Reward Points dramatically, which as noted before had been allowing for free play for many, and flat out "profits" for some. Many of those who could no longer play for free simply quit. The fact that they implemented this almost at the exact time of Conte's Inferno simply exaggerated the effect of both.
3) Many of the coaches who quit playing because of the above reasons stuck around the forums and viciously attacked the product. What had been a relatively nurturing place for newbies became an acid bath. Very few new players stuck around the game.
4) Fox Sports bought WIS. What should have been a good thing for WIS turned into a disaster. Fox pulled programmers and other resources to their "name brand" products, leaving GD to fester in a state of disrepair. WIS is not the red headed step child of Fox - it's the fat nerdy friend of the red headed step child.