They could not handle the losses. Some could not handle losing money at all, others chose to not manage risk and the big losses ruined them financially and they had to stop trading. Or the big losses ruined them mentally or emotionally for trading and they could not get back in the game after the damage was done.
They had a overwhelming desire to be right all time and could not handle being wrong. They could never grasp that trading is not about being right personally but instead simply being on the right side of the market price action.
They thought they were smarter than the market and all the other traders even though they were just newbies. The market showed them how counter-intuitive and irrational it can be.
They did not do their homework before they started trading money. Their education was in the moment and they were doomed from the start. If new traders are too lazy to read numerous trading books, study charts, study legendary traders, understand the risk of ruin, trader psychology, and test systems they will never make it. The hardest part of being a new trader is to even discover what you do not know and find out where to start.
If you enjoy the game of trading and the markets you may make it because you may not quit when you understand the work ahead. If you are doing this 100% for the money and think you will go take some quick easy money from all the professional traders with little effort, you are going to have a bad time. Passion for trading is the you must have to take you over the learning curve to profitability.
The new trader has to first focus on finding out what the right questions are then they have to go find the right answers from the right places.
Many new traders come from other professional fields and just can not get back to the beginners mind required to learn a new field of expertise. They want to be instantly knowledgeable and respected as experts and that is a long process that takes years of study and real time trading success.