Voice acting could be implement on scripted games like witcher or fallout. Because its depends on player interaction. For example in witcher 3 there is a cutscene which says you have to find ciri or Eredin get her in a matter of time, after the cutscene ends i slept with all the woman i came across and did tons of side quest. And that didn't change anything when i went to ciri because its scripted and there can be voice acting.
But in mount and blade even a simple question like "Where is x lord" needs lots of different answers. Or imagine talking with marshall and he says "We will attack x castle with y lord".Because its an unscripted game that doesn't wait for player interaction.
You need to dub every city with every lord combination which is pretty expensive and unnecessary.