3. Why was the International System of Units adopted?

Explanation:

Because the units are specifically designed in such a way that anyone, anywhere could reproduce them. They are based on scientifically developed definitions. For example the meter is defined as

Current definition (1983): The distance traveled by light in vacuum in second.

That does not look like a great improvement, but it is certainly better than the distance of 1/10,000,000 of the distance between the meridian going through Paris going from the North Pole to the equator. How would you even measure that without a guess somewhere?

The subunits are also much better. 1/10 of a meter is much easier to dial with than a foot or a yard which is very arbitrarily defined.