It's not that outrageous to suppose that the houses would be VERY reluctant to set a precedent of this kind. "Sure, the WoL did some amazing things, but if we let one outsider in, pretty soon they'll all be banging on the gates of Ishgard to sign up!" Never underestimate the lengths the privileged will go to to cling to the status quo! They're happy as they are, and in their mind any change is likely to be bad for them, rather than good. The Houses are likely particularly antsy, given that the greatest pillar their privileged position in society rested upon (that they are descendants of great heroes) has been taken away.
