The US government as a company belongs to you. Well, not totally, but at least you're kinda like a shareholder. And as such, you're entitled to them using the funds you provide them with well. Them simply declaring that they will only buy from this provider or never buy from that provider requires oversight, or it becomes a cesspool of bribery and corruption.
There has to be oversight because, well, would you, as a shareholder, want your CEO to buy his supplies from a company that just happens to be owned by his spouse, no matter whether there are better offers? Or would you want your CEO to avoid buying a best offer because he doesn't like the vendor for some personal reason?
Don't get me wrong, if there is a good reason not to buy there, I'm glad they don't. And it's sad that the vendor has to second guess their actual motivation instead of you, the owner of the company, doing it.
That's actually your job. Selecting one of the two offered applicants for some management position every other year isn't enough!