It's because of how recent the "invention" of electricity is compared to ideas like distance and weight.

Weights and lengths were things that all sorts of civilizations had to deal with (even the ancient ones) and so those civilizations all individually developed "units" that were convenient for them (or borrowed from their neighbors). So, after you get good and comfortable with your units over a few hundred years, when you meet some other country that says "Hey, your units are weird, why not use ours!" you tend to be like "No way, it is your units that are weird! Buzz off!".

Electricity, on the other hand, (which was invented and commercialized in the last couple centuries) was basically a brand new thing. That meant that the first country that had it was like "Hey, this is how it works and these are the units we use!" and everybody else was like "Okay, cool! Sign us up!".