If usability is not in the request for proposals, it can be difficult to add it after contract award, and bidders are unlikely to propose it for fear of adding cost and not being selected. RFPs should require not only that the end product be usable and that usability be tested before launch, but that the project follow a user-centered design process throughout the lifecycle. For information on the human-centered design process and its management, see (for example) the following ISO standards:
ISO 9241-210: Ergonomics of human-system interaction -- Part 210: Human-centred design for interactive systems
ISO/PAS 18152: Ergonomics of human-system interaction -- Specification for the process assessment of human-system issues
I'd like to see this be a policy for all agencies and all procurements that involve anything other than pure COTS or GOTS.