Annie Mitsoda said:
A distilled reason is Because It Is Expected. There's been a rebellion against the silent protagonist (Link is perhaps now the only holdout), but really - protagonist voice - and full voiced dialogue in general - has now become a fully expected part of games. If you DON'T have it, there's the danger of an audience believing your game looks and feels unpolished (correct me if I'm wrong, but people BAWWW'ed over the fact that the protagonist is silent in Dragon Age).
Mind you, I'm not saying this is right or that I agree at all - I'm just saying it's the prevailing opinion in many circles, and one of those circles seem to be the publishers, so there you go.
Is the ubiquitous Gordon Freeman of Valve's Half Life not a Silent Protagonist?
And is the Half Life series not considered by many PC Gaming magazines to be the "best" series of games in history?
The silent protagonist as unpolished thesis doesn't hold up when you look at games like Half Life and Bioshock, both games sold massively and have not been baww'ed over because of a silent protagonist. Oblivion, KotOR and Fallout 3's protagonists were all silent as well, so the same can be said for RPGS.
The problem is likely more structural than anything else- Mass Effect has cinematic dialogue sequences in which the protagonist is shown from a third person perspective and so is removed from the player- the player controls the protagonist inasmuch as what the protagonist says, but the characterisation of the protagonist is pre-determined, manner of speech, attitude, etc.
In all the other silent protagonist games you embody the protagonist even in dialogue, which is usually in first person; in the isometric rpgs the system is slightly different, but one could argue Fallout 1 and 2's talking heads system worked similarly in that for major dialogues you took the place of the protagonist in the conversation rather than the character model on the screen.
When dialogue is conducted in first person there is a tendency to voice the protagonist in your own voice in your own head, so no need for voice; you are the protagonist in the dialogue and therefore do not need to hear another voice to suspend disbelief. Enter Mass Effect and the 3rd person dialogue system in which you are taken out of the protagonist's head and enter "choose your own adventure" dialogue system which then plays out cinematics based on your choices; you are no longer the protagonist therefore s/he needs to be fully voiced for you to continue to suspend disbelief.
(A silent character model from 3rd person perspective standing in dialogue completely silent while being responded to would look completely off.)
So I'd argue it's more a structural element of how dialogue is implemented that effects whether nor not full voice acting is required.