D&D: We thought it would have been very confusing to the audience if Robb Stark married a Lannister Bannermen. We needed to make the story very straightforward and clear and this of course incluedes character decisions. Making her a foreigner would make it less a mistake from a northener point of view, and we chose Volantine because we don't know anything about that place anyways, so it gives more room to freedom. All the changes also had the intention of making the audience more shocked once red wedding arrived. Stabbing her baby I mean. That was not necesary but we were looking forward to do it.
Next question: Why didn't you care about the fact that you wouldn't catch the last 2 books in time from the very first beggining of the production, and thus making the whole preparations that anyone would have known that it required, like preparing Dany's twist way sooner, or developing a plot for NK that made a little sense at least?