1/ Lack of Discipline— a) Not sticking to one strategy, b) not respecting your trading rules, c) not sticking to your plan.
2/ Lack of knowledge—Inadequate understanding of the market, trading strategy, financial instruments, and fundamentals can lead to poor decision-making.
3/ Not having a mechanical trading system—This leads to emotional trading. Decisions driven by emotions like fear and greed rather than rational analysis result in significant losses.
4/ Overtrading—Taking many trades will affect your decision-making ability and lead to poor decisions.
What do you mean by "mechanical trading system"? Do you mean a trading bot? I can imagine how the financial markets would look like if all the traders were using automated trading bots.
There's no way everyone on the market to make a profit and nobody to lose money. Somebody has to make poor decisions and somebody has to fail at timing the market. All the things you mentioned in your list are true, but so what? There will never be a situation on the market, where all traders are perfectly disciplined, perfectly competent and have a perfect strategy.
I don't consider overtrading to be a bad thing. The day traders are gaining experience by trading more, even if some of the trades aren't profitable.