1. The lack of knowledge or proper knowledge.
2. Poor/improper training on what the trader knows to achieve a trading system.
3. Lack of proper money and risk management.
4. Lack of coordination of what the trader knows and trains about to form a viable plan with them. And finally,
5. Poor trading psychology.
I also think the point number 3, 4 and 5 are the big problems that make many traders fail. Usually they do not manage risk well and do not do analysis and make a good plan about how many assets they want to trade, where they will enter, when they will exit and how much profit they will get. If they have made this plan, they just have to follow it because everything has been planned.
Point 5 will happens because there is no plan from the start so they don't know when to take profit. They keep hoping the price will continue to rise until the price has fall and only get a cut in profits.