Without content that the vast majority of players enjoy people won't spend money in the gem store.
I've only been playing GW2 for a few months but, I'm loving the open world content and storyline quest.
I played WoW from Vanilla through till MoP and since have dipped my toes into a lot of MMO's looking for one that gave good storytelling and open world PVE content but, none really scratched that itch.
Sadly I had avoided Guildwars and GW2 because the name made me believe it was all about PVP (I like PVP but as a fun option not the mainstay of a game) then I saw an article about the open world content of the game and thought I'd give it try and I have to say that IMO GW2 does this type of content better than any other MMO I've tried