It was specifically written after the civil war so that the South could still technically have slavery, they just have to criminalize and convict a person first. It's how we get the 'war on drugs' where even though black and white populations smoke around the same amount, black populations were criminalized FAR harsher, and sent to prison far more often. It's exactly "Rules for thee, but not for me" on a grand scale where white people would likely 'get a pass' for their first arrest. That grace was not extended to black populations, and so we're back at having slavery with extra steps.
It's a nice thought that it just happened over time by accident, but in reality it was planned like this.