Surprised that this totally blows past the chemical reasons why not. There's an excellent answer by shigeta on the Biology Stack Exchange[1] to this very question.
His major answers are that electrical conductivity is likely to be a huge problem for the nervous system, and that iron and aluminum readily oxidize (aka rust). Iron oxidization is useful in hemoglobin but would be problematic in your bones, and while a body could be designed with conductivity in mind, it would at minimum also take a reworking the nervous system.
His major answers are that electrical conductivity is likely to be a huge problem for the nervous system, and that iron and aluminum readily oxidize (aka rust). Iron oxidization is useful in hemoglobin but would be problematic in your bones, and while a body could be designed with conductivity in mind, it would at minimum also take a reworking the nervous system.
1: https://biology.stackexchange.com/q/9419/4101