A somewhat more interesting problem is we live in a centrally controlled economy with many centers. There's a paperwork storm downstream of thousands of local building codes. Most are minor variations on minor details of the NEC, but it would be a huge job to harmonize and metric-ify them. For example where I live, politicians "had to do something" after someone died in a pool electrocution decades ago, so our local code is NEC plus a microscopic tightening of 1950s NEC swimming pool regs (which ironically are probably looser than 2016 NEC swimming pool regs, negating the politicians intent from decades ago).

Anyway at least for awhile if you want legal electrical work done, you're going to have an incredibly expensive and dangerous cutover. Its likely that cutover to metric related mistakes will cause as much property damage and death as "X" years of not harmonizing under one system. Where "X" is probably many more years (lifetimes?) than you'd expect.