Meh I think Guild Wars started dying a few days after NF came out.
Wait till GWEN comes, and alot of people who took a break come back ^^
No new players will buy GW:EN, though, cause they need an other chapter for it...
As for the reasons... After you beat the storyline, it's either PvP or quitting. In the past, this was prevented with other stuff in the game, but they kept being altered and nerfed to the way they are now - not fun.
So yeah, most people who beat PvE most likely went to do PvP.
The difficulty of the game also might have to do with it. Missions like Thunderhead Keep used to be 'People stoppers'. Most people got stopped there, cause of the difficulty, and were forced to team up and creat good parties to beat it.
Now, that is no longer needed, as you can just fetch some henchies and beat that mission with the heroes, who have just the builds you needed. The rate which in people beat the game has increased, and the stuff to do after the game has been nerfed.
But I put my hopes in GW:EN, and hope people will start playing then again! (Since GW:EN has cool stuff to do next to the storyline, like the mini games, boredom shouldn't be a problem.

)