I suspect the CNC costs in the US were too high and they couldn't get a steady supply of cases if orders peaked. Not that I'm an expert, but I suspect it's at least a six-axis CNC unit that's required.
You'd be surprised, US manufacturing costs have become much more competitive over the past 5-10 years. Perhaps Apple is missing the expertise in the USA to manage manufacturing that has been built up like an army in China. Realistically, it comes down to the CEO, aka ex CFO's addiction to Chinese Manufacturing and Labor costs and loose environment restrictions.