Re: Why a kilogram?
The question we need to be asking the French post-revolutionary government is, having fixed the metre as the unit of length, why did they decide that a gram should be the weight of a millionth of a cubic metre of water? Why not a thousandth, or one? If they'd gone for a thousandth, things would be more consistent now.
My guess is it was a practical decision based on the use of balance scales. A set of weights for a balance scale would increase by powers of 2 or 3, which is easy enough to work with as long as the smallest weight you commonly need to weigh is a whole number. If you need to weigh stuff less than 1, the decimals get unwieldy: 0.5, 0.25, 0.125, 0.0625, 0.03125... (or, heaven forbid, 0.333, 0.111, 0.037, 0.0123...). Or you go back to labelling your weights as 1/2, 1/4, 1/8... which somewhat defeats the purpose of having a decimal system.