They are not intelligent enough. Their psychology is all wrong for trading. They underestimate the skills required. They overestimate their own abilities. They risk money before they have learned how to trade. They over-complicate their charts. They over-simplify their charts. Their heart is just not in it. They lack the necessary patience and perseverance to succeed. Their losses are too large in relation to their wins. They have no tried and tested strategy. They are not disciplined enough to stick to their strategy. Because ultimately, trading is gambling, and sooner or later, all gamblers lose.