Not to mention, they're going to pass the books by the end of this season, no?
So, basically, if you're to believe the rumors, GRRM told the producers how the series is going to end (Outline maybe?...I know that's how I prep my "books" personally) and they are basically moving towards that end in the way they best know how.
Remember, this is a TV show, so at a certain point, having "too" many actors can be a problem. It's probably the reason Sansa has such a bigger part to play (because the actress they have playing her is talented and on contract).