When you have a cash shop, you suddenly have a conflict of interest. For every item made, person hired, or unit of time spent on development, there will always be some bean counter that's asking "can we allocate this to the cash shop?" thus removing content from the game that we payed for through subscription.

The only things that should be in the cash shop are administrative things (race change, name change, etc), and things you can acquire in game (old event items). That at least encourages the devs to make cool event items, in hopes of selling them when the event is over.