I would expect that when push comes to shove most of the candidates from rounds 1 and 2 were simply unsuitable; either they were non-serious/joke entries, physically unfit, or had no relevant skills.

There's a reason we send only the best of the best into space - it's phenomenally mentally and physically demanding, and a manned Mars mission is about the most dangerous and challenging expedition any group of humans will have ever undertaken.

There's no room on that boat for people who can't pull their weight.