Math.
How many people have basic understanding of NFL football, experience in at least one aspect of the game (e.g., cap, draft, coaching), are within the 'NFL family,' willing to work long hrs. That's just a few. I'd be willing to bet a lot, at the next GM opening there aren't 25 candidates the owner would consider. It's a closed community requiring some level of specialized knowledge.
How any particular GM gets the job I have no idea. At some point it comes down to the owner's gut feel.
To me, it's like a new HC. In any given season there are 5 or less newbies getting 'buzz' and 3 or less experienced, successful HCs available. If none of experienced HCs want the job and a couple of newbies have 2 or more offers, AND if none of them are clearly superior, compromises must be made.