UPDATE!
LINKAGE
It would appear this question has been asked by the CSM, but it needs balancing.

They don’t release what they have planned for future expansions until it’s getting near that point, so nobody would know except for CCP themselves. As for why they aren’t there, that would be an unknown. It is possible that it would be a game unbalance problem, or it may just not be asked for enough by the player base. Maybe this is something that should be put on the forums, unless it’s already there. Sorry for not having a definite answer for you.