It is my understanding that Turbine has stated that it's difficult for them to do this because it's an instanced area. However, that doesn't make much sense. Here's why:
Freeps have a vaultkeeper inside GV in the Moors.
Freeps have mailboxes at their houses, inside instanced areas
Freeps have a vaultkeeper inside the housing instances, at the center of the housing area.
So, they have proven three times over that it is, in fact, possible to have mail and vault services inside an instanced area.
That being said, though, it would mean they'd need to set up a separate mail/vault system than what they already have freepside, which may prove a little challenging. Even with that fact, though, I find it still questionable as to why it isn't possible.
~Main characters~ Arkenstone: Sansanor, 100 Hunter; Mathilanakh, R11 Warg