Little history
I dont have any date but in some past time, some years ago , almost all the planet wanted to switch to metric system. So the American governement started a joint program with Canada to convert to metric mesurement. At some point in that process tha Canadian seeing American not changing asked why??? The answer was ... to expensive... what th f$%?. So history says US started the conversion and now WE are paying because US did not convert, every time we buy mesured item WE have to convert to SAE to get our order trought (comercially speaking). And YES metric is alot easyer, when searching for the next socket because the one you have is too small, all you do is add 1mm. With SAE it depend on what grade your kit is 8th, 16th,32th and sometime 64th, so to get higher than 1/2 by 1/16 it is 9/16, buy by 1/8 it's not 10/16.... it's 5/8. All I'm saying is you need math school to understand.
No offense to people using SAE, I do use it when framing thing, because wood is sold in SAE. But when cutting for my project I use Metric.