Apparently it was and still is ludicrously expensive to make the switch so they don't bother.

I just learned that my country went metric right after WWI when we regained our independence because the existing units of measure were a mess. We had various units used by the empires that ruled over us, like Austrian miles, Prussian miles and Russian versts. We had several currencies in use too. It sounds like the unification process required quite a bit of effort.

On the other hand, I think these might have actually been easier to implement back then, when the country was wrecked by the war and had to rebuild anyway.