The metric system was designed to, as much as possible, use scientifically measurable things for its units and that the "bigger versions" of units are just multiples or fractions of 10, 100, 1000, etc.

Most of the measurements involve water. A litre of water on earth at sea level (average conditions, admittedly) weighs exactly 1 kilogram and in a square shape would be exactly a box 10cm by 10cm by 10cm. Similarly a square of water 1m on each side is 1000 kg, or a ton. So now you have a way, given only the definition of a meter, to get the weight of a kilogram or vice versa.

Whereas the imperial system has a "foot" defined by... I'm not sure what really, but it divides into 12 inches, or can be combined by 3 to make yards, or 5280 to make miles. Eww, what terrible numbers. In metric a kilometer is just 1000 meters. A kilogram is 1000 grams. Etc. Nice consistent numbers.