Risk management - Rather than looking for one big trade, the way a trend trader might, the scalper looks for hundreds of small profits throughout the day. In this process the scalper might also take hundreds of small losses during the same time period. For this reason a scalper must have very strict risk management never allowing a loss to accumulate.
Retail investors are prone to psychological biases that make day trading difficult. They tend to sell winners too early and hold losers too long, what some call “picking the flowers and watering the weeds.” That’s easy to do when you get a shot of adrenaline for closing out a profitable trade. Investors engage in myopic loss aversion, which renders them too afraid to buy when a stock declines because they fear it might fall further.