Building an MVP and iterating requires building to a (changing) vision. Contractors have an incentive to stick rigidly to a narrow, fixed specification. If you can build that spec and task it out then you may get good results, but just telling a contract developer your vision and sending them off to build it has a strong chance of ending in (expensive) tears.
I am a contract developer. I work with clients to elicit their requirements and have seen this first-hand - the projects that turn out best are the ones where I'm working with someone in-house who has development knowledge, even if it's just to manage and review.
MVP+ stage requires a lot of iterative development. Ideally, you'd want to keep the people who are helping with that, because they'd know WHY things are done the way they are. They'll be aware of the failed experiments and previous iterations. You don't want future employees making the same mistakes. The goal of the MVP+ stage is to learn as much as possible about your market and product. Letting that product learning walk out because they were contractors is probably a mistake.