I think traders actually fail for a huge variety of different reasons, of which the “Big Three” (although they mutually overlap to a considerable extent) are probably …
They never actually develop a system/method which has a genuine edge (positive net expectation) in the first place;
Undercapitalisation and excessively risky position-sizing;
They don’t have, or develop, enough judgement/discrimination to decide by whom to be guided.
I think the first two of those “Big Three” perhaps fall under the more general heading of lack of education/understanding about the statistical and probabilistic realities that necessarily underlie all successful, profitable trading, while the third probably relates more to the nature of the industry itself, the people it attracts, and the fact that the failure-rate with all forms of attempted self-employment/business is pretty high, and misinformation and misguidance (some of it actively incentivised) is all over the place, especially online where there’s absolutely no quality control over publication.