Because DA:O was soooo generic with it's choices; c'mon that premise is ridiculous. It is all about execution. They've pulled it off with the Warden and it can be done again.
It actually was pretty generic. Sans rose colored glasses, we can surf through the Origins forums and find plenty of "Why don't they acknowledge my race" or "Why don't they address me by my name" threads. Through the vast majority of the game, you are "Warden". Then there's the whole "prop" angle, where you're more something for the NPCs to look at than an actual participant. The "but we can assign our own meaning to the chosen dialog" falls flat simply because you can't. You can tell yourself that you did, but I'd bet there's more than one post that claims they meant a line as a joke, but the NPC went postal on 'em.
The reason for that latter is simple, really: A silent protagonist doesn't get any more NPC responses than a voiced one does. How many choices led to the same responses? The conversations are scripted to have preset results, and in a case where you have more than 3 lines to choose from, the NPCs may well have the same response tied to more than one. Why? Limits in technology. If they ever make a game that reads your intent when you choose a line, and then comes up with a custom response to it, instead of preselected responses, then "but we can assign our own meaning to the responses" will be relevant. Otherwise, you are still limited to what the writers give you.