This might be a good thing. If all the voice actors haven't done their work yet then that means the writing has more flexibility to work itself out for a while. If I'm not mistaken, Oblivion did their voice acting too soon in the development cycle.
My guess is that the VA is one of the last things they do since so many things have to be in place.
All the quests have to be done
All the NPCs and their links to quests
All of the possible scenarios thought through that would impact an NPC response
etc. etc.
It's got to be a huge job keeping track of all the dialogue and make it fit together.