In fact, no country on the planet has voluntarily switched to metric, because for normal human beings it's inferior to, say, the Imperial system.
The only thing "better" about it is the conversion of units, but in reality almost nobody outside of government bureaucracy and engineering/science converts units often, if at all.
In every other way, the Imperial system is better. It has major units closer to those natural to use, and tends to be binary instead of decimal, so that even sub-units are generally closer to what someone needs at any given time.
Even when it comes to conversion, the main problem is bureaucrats too stupid to be able to do math if it's not a simple decimal system. And it's a damned shame that our numeric system is decimal, instead of some power of two, like hexadecimal, or at least base 12.
Because of this, each country only "switched" when the soulless state bureaucrats pushed their corrupt political class to force people to switch. Otherwise, people would have continued using the system that works better for them. In the US, there is no state power to force people to switch...fortunately.