Well, a few things.

1) a business can charge what they can get, simple
2) a little more in depth, with the lack of content this game has, I too am beginning to question the $15 subscription fee per month as worth it. Basically being maxxed out on content right now, 2.2 will come out with 3 new bosses in Coil, and Leviathan Primal. So, 4 new boss encounters seems interesting, lets say it lasts 2 months even. The problem is, with 2.3 patch, it will just be Crystal Tower downgrade, so I can probably skip that. So I will have to wait an extra 6 months for new Coil, which leaves a 4 month 'gap' window that I just cant really justify paying a subscription, let alone $15. Also I do not currently see the development I would expect for a game with million users paying monthly fees.

tl;dr content and development seems really lacking for a $15/mo MMO