We don't know that information is conserved. It is postulated by the linear unitary time-reversible Schrodinger/Dirac equation, and assumed by the Many Worlds

We don't know that information is conserved. It is postulated by the linear unitary time-reversible Schrodinger/Dirac equation, and assumed by the Many Worlds Interpretation, but contradicted by the non-linear probabilistic time-irreversible Measurement Axiom of conventional QM. Some people (esp. Penrose) postulate that gravity can collapse the wavefunction, and hence destroy information (esp. Black Holes). The Black Hole Information Paradox is a hotly contested current topic.