One of Leisure Suit Larry's complaints is vendor lock-in. No matter how much you try to avoid lock-in you will have some degree of lock-in. Whether is proprietary products with proprietary file formats, a query languages with non-standard syntax and extensions (looking at you Larry, you lying scumbag), familiarity with the UI, or something else you end up with a degree lock-in with any product. Whether DOD selects AWS, Azure, or someone else they will have some degree of lock-in even with Larry's Minions.
The main advantage with a sole source cloud contract is to have one point of contact instead of many possibly incompatible systems in use. One way to drive up the costs of a system is to have parts from many vendors with no one have the responsibility to make it all work. You saw with the Obamacare rollout, no one vendor had the authority to force the subvendors in line, so you ended up a morass of non-functioning, incompetently designed and written systems that did not work together. Note of the chief offenders was Larry's Minions.