As Professor Grant pointed out, it's entirely appropriate that the burden of proof be on the accused to prove, on the balance of probabilities, that they were in such a state. It's information and evidence in their hands, not the hands of the prosecution.
Furthermore, it's not considered to be a normal state, that kind of state where the person is not operating with their body and mind together. We require that the accused be the one to bring forward evidence to prove they were in such an unusual state.