1. In poker you play with individuals. Your winning comes from fishes and suckers, who have worse skill than you. In trading you are deal with institutions, and the money which can move market is controlled by smartest people, who have better research tool and better information sources than you, and are a lot smarter than average traders.So your opponents in trading are a lot tougher than your opponents in poker.
2.In poker you bet by wager, so you only risk what you put on table.If you get +50% probability( or +EV) on your wager you win.
In trading you risk all your capital. You could lose all your capital even you have 90% probability.You could win 9 out of 10 times, the one you lose could blow up your account.
So you need to set stop to protect your capital.And after you set stops, your probability are dramatically lower than before. Let's say you see a pattern that has 70% probability moving up.But market doesn't always move straight up. It goes up and down and up. So in 10 times, 7 times market end up moving up, Of which, 4 times it goes straight up, 3 times it go down first, and trigger your stops. So you end up only win 4 times out of 10.Your probability, after you set stops, become 40%.