Going all in means putting all your eggs in one basket. If the market turns, your account could bleed heavily, or even go to zero. In trading, no one can claim to be 100% accurate, going all in means ignoring risk.
Losing streaks are hard to recover from: If you lose, you run out of bullets.
In trading, winning and losing are common occurrences. Even strategies with high win rates can experience losing streaks. If you lose everything on an all-in bet, your funds are destroyed, and you won't have money to trade next time. Good position control at least gives you a chance to recover.
Market fluctuations are hard to predict: Black swans can appear at any time.
Who can predict the markets? Black swan events, such as lack of liquidity or price gaps, can lead to disaster if you're all in. Normal fluctuations could also trigger your stop loss; going all in amplifies risk.