I think it's something to do with the money they earn. Don't ask me how it works, but I remember reading a few years back that they take a hefty chunk of money from East Coast for operating York - Kings Cross. I think they get a bigger amount of money than what fares they actually take for their services alone. I think the same is true for Hull Trains and is the reason why the West Yorkshire service stops at Doncaster. I think it's all divided up proportionally. If it went to Newcastle, I think the Rail Regulators wouldn't allow it as they would be seen to take too much money from the main ECML route, hence the random start point of Sunderland.