For radiant scenarios that can pick out any NPCs to fill a role, it's easier for Bethesda to reuse voice actors for different characters so that they can reuse that dialog. Whether they get more voice actors or have their voice actors use different voices doesn't matter, they still have to record radiant dialog for most every different voice in the game.

Not that I'm saying I wouldn't want every NPC to have a different voice, but Bethesda has a very good reason for reusing voice actors for generic NPCs. I'm hoping Fallout 4 has a nice big cast and some solid voice direction, and the way Brian Delaney talks about his experience voicing the protagonist has me really excited.

NV had actors that could change their voice. They also didn't sound so unique that you heard them everywhere.

Paul Eilding voiced nearly all the old male NPCs and they sounded EXACTLY the same.

http://vignette2.wikia.nocookie.net/fallout/images/c/c1/Fo3_NathanVargas_YoullSee.ogg/revision/latest?cb=20130728233025

All the old women sound like this.

http://vignette4.wikia.nocookie.net/fallout/images/5/5a/Fo3_MargaretPrimrose_ItsHisBuilding.ogg/revision/latest?cb=20130729173735

Many of the males sound like one of these.

http://vignette1.wikia.nocookie.net/fallout/images/c/c4/FO3_Armitage_ThatsCloseEnough.ogg/revision/latest?cb=20130723220428

http://vignette4.wikia.nocookie.net/fallout/images/a/ad/F3_Joe_Porter_get_a_meal.ogg/revision/latest?cb=20130705135146

They didn't change their voice at all and their voices sound so unique that it gives the impression they're everyone in the CW.

Bioware had a really good solution for increasing voice diversity - community theaters.