The reasoning as I understand it: Vaccine production is too complicated for open access to work well. There is a significant risk that something goes wrong and the vaccine factory has to shut down, so it is better for that factory to be producing a vaccine they know exactly how to do. Oxford partnering with AstraZeneca ensures they can work closely together to get the production right.
Could they have made a non-exclusive deal? Probably, but (a) AstraZeneca might not have been interested in that case and (b) Oxford might not even have the capacity to work with more than one partner.
Could they have ensured better incentives to ramp up production quickly? Definitely, but that seems like a decision mostly independent of making an exclusive deal.
Bill Gates himself addresses this question here: https://youtu.be/Grv1RJkdyqI?t=558
The reasoning as I understand it: Vaccine production is too complicated for open access to work well. There is a significant risk that something goes wrong and the vaccine factory has to shut down, so it is better for that factory to be producing a vaccine they know exactly how to do. Oxford partnering with AstraZeneca ensures they can work closely together to get the production right.
Could they have made a non-exclusive deal? Probably, but (a) AstraZeneca might not have been interested in that case and (b) Oxford might not even have the capacity to work with more than one partner.
Could they have ensured better incentives to ramp up production quickly? Definitely, but that seems like a decision mostly independent of making an exclusive deal.