"Only 3% of players have fully upgraded all the inventory space on all characters and maxed out bank space, so we're not going to add space to housing" -Zenimax
#ZenimaxLogic
That's perfectly sound logic.
We all have a giant wishlist of housing features, and ZoS knows storage is one of them. In a project like this, you have to pick your battles so you can eventually finish the first release. If 97% of the userbase doesn't have a pressing need for this feature, and there is already an existing feature that provides similar functionality, then it becomes much less of a priority.
Until many more players start maxing their inventory out, then they should prioritize features that will deliver more bang for the buck and positively affect the most players overall.