Fleet sales.
Companies who buy trucks by the hundred would like to have the same point of contact that they've had since forever on their lease/purchase deals. As such, if they have a GMC contract they will keep negotiating and renovating it. If GMC were to close, those contracts would have to go to tender again and further options evaluated At best, this ends up with everyone just switching to Chevrolet, which is wishful thinking at best. And no, they can't just deliver Chevrolets. GM has already been sued for similar things in the past.
This is also the reason GMC survived the GM restructuring. It was it or Hummer, and GMC had at least a guaranteed quarter of a million sales a year. Everyone else has mentioned the differences on the consumer side, so add those to the list.