* SPOILER ALERT *
(If you haven't watched last night's GOT episode, discontinue reading)
Once you establish that the series is going to be about various factions competing for the Iron Throne following Robert Baratheon's death, it makes sense to get Ned Stark out of the way. He's a guy with no ambitions for the throne, and he isn't going to be pushed into making a claim for it. By moving control of House Stark to Robb, you get the setup for having the North backing their own King.
Plus, on a more meta-level, the world that Martin created didn't have a place for people like Ned Stark -- his loyalty and honesty are incredible handicaps when it comes to dealing with other nobles and lords who lie, cheat, and murder without a second thought. Ned is a soldier and a terrible politician, and he pays for that fact with his life when he steps into that arena.