That's probably one reason... having to record the lines out of context and without other character's lines to work off (let's not forget that most NPC lines are reactions to things the player says, which are not voiced at all) makes it probably difficult, especially if the voice actor is not familiar with the franchise, genre or possibly video games in general, and maybe doesn't even understand what he or she is saying.

But really, if you thought Oblivion's ENGLISH voice acting was bad, you should hear the German version. ;) But the translation was completely messed up, anyway, including all written text (like item names which had to be abbreviated because the translation turned out to be too long for the inventory menu to display - only that fans managed to fix this with mods, so why couldn't the people who translated it do it?). Anyway, I've stopped playing the German version after one hour and never played any Elder Scrolls or Bethesda games in German again since then. So yeah, in comparison, the voice acting in the English version is really not that bad. ;)