If you mean why is quantum mechanics probabilistic? That's because it's the best model we have to handle many of the momentary variations in physical systems. But I'll say that determinism could still be true even if we can't make predictions. But equally, I believe that determinism can be false even if the observable universe seems well ordered. Chaos doesn't just mean random things happening without a cause that could be found. It just means that not all chains of causality can link backwards and forwards perfectly in time (i.e. some chains of causality maybe emergent at best).