[Dn_Ab]

Simulating classical systems results in slow downs that are only polynomially with respect to system size while quantum systems slow exponentially. Consider that a quantum system of n qubits would require a classical system able to cope with 2^n bits. But you might be right since there is a (remote) possibility that classical systems can efficiently simulate quantum systems. However, it is generally believed that quantum computers are more powerful than classical but not strong enough to solve NP-complete problems.