Carbon is a versatile molecule. IT can have many different material properties depending on the environment and the types of molecule it's surrounded by. More importantly it can form single, double, or triple bonds. This is part of the reason for it's versatility. It can bond easily with 4 Hydrogens, more than any other PROMINENT molecules found in the body. It can attach various functional groups to itself. I'm just listing things here, not really reaching a conclusion. I think part of the reason we're not made of silicon is because it's not electronegative enough to give much versatility to our body. IT won't have the ability to attract as much electrons as strongly as carbon. Plus, it's a metalloid so we'd be conducting electricity!