Why are "eye for an eye" laws no longer used?

Eye for an eye, or that type of legal system, has progressed as almost everything has throughout time. It's become more doctrined, a refined process in most countries to allow "due process". If you don't know what due process is, read about it. When 'eye for an eye' was the legal doctrine to determine punishment people were fighting wars using spears. When, if it is even allowed, to end someone's life even if a criminal one at that, it's best to do so as humanely as possible. Crime existed in the ancient days, so I wouldn't say going back to that legal system would prevent or "plummet" crime rates today.