People lose big money because they do not even understand the importance of using stop loss in a trade. Come to think of it though, how will someone who is totally and absolutely ignorant of what they are doing even be able to understand how to make use of a stop loss to make a good trade.
People these days are always in a haste to want to make money and that makes them rush the process of learning, practising and trying as much as possible to be able to do things right before taking the real step. Someone who is just entering the market blindly in most cases, will always buy at the wrong positions, sell wrongly and never make use of stop loss, because they are ignorant.