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.
However it’s also interesting to note that one third of the round two candidates have dropped out since December, whether due to not passing the medical or for personal reasons. I’m not very surprised at that - I think that as the rounds go on, more people will have second thoughts (maybe I will, can’t rule that out), and it is a big commitment to make.