Because determinism fails to accomplish the major goals of philosophy, which are
1) Epistemology -- the study of how we know things. Because we cannot measure that thing that causes us to remember, we cannot use determinism to study how we know.
2) Metaphysics -- the study of the structure of the universe. Science has gotten us to quantum mechanics, but Heisenberg's Uncertainty Principle limits our ability to measure very small things, and the vast gulf of space limits our ability to measure very distant things, so we cannot use determinism to study either extreme of the universe's construction.
3) Ethics -- the study of how we ought to act. Because we cannot take all variables into account at once, we cannot accurately predict the outcomes of our actions using determinism.
4) Logic -- the study of how we reason. While determinism follows the rules of logic, logic does not always follow the rules of determinism. Determinism postulates that all statements that are made are either true or false, whereas many branches of logic -- particularly 'fuzzy logic' -- allow for truth values to be any real number between zero (untrue) and one (true). Thus at least one entire branch of logic lies outside the realm of deterministic analysis, and the complete failure of determinism to function as a philosophy is complete.
