In a rough order of the reasons why:
1. They trade too large a size, as if the market is going to be shut off, because they are in a rush to make lots of money
2. A trader never taught them when to buy, sell ie they do not really have momentum or direction odds in their favour
3. They never learnt how to place a stop loss which is customised to every market condition ie volatility based stop loss
4. They wrongly think placing a stop loss far away, so they are more likely to win, means more profits. In fact, yes they may then by right 93% of the time, but they will lose more money than they make when they are wrong.