Because all the conversion factors are decimal and we are used to using decimals. The other feature is that the metric system has some unit conversion defined, a cubic centemeter of pure water at a certain temperature masses 1 gram, so if you had a very good metric ruler you could create other measurements.
You still may need a dictionary because the prefixes (for example pico) are in latin. The units, for example, newtons which measure weight, although kilograms are used generally (in error), can be just as confusing as feet, inch, and yard. The one nice thing is that you can have a third of a foot (4 inches) but a third of a meter is a repeating decimal in the metric system. In fact any division by numbers not powers or products of 2 and 5 are repeating decimals in the metric system. It was too bad that when they created the metric system, based on science rather than human measurements, they did not use another base, like 12, which would have made the system more useable. Ten fingers is just as arbitrary as the width of the kings foot.
It is more easier to convert units in the metric system, since all units in this system are expressed as powers of 10. So, if you want to scale up and down, all you need to do in metric system is to move the decimal point. You don't need math to convert 1375 meters to Km. All you need to do is to move the decimal point 3 places to the left side: 1375 m = 1.375 Km.
Unlike metric system, the U.S. system is more complicated because if you want to scale up and down, you need to memorize all kind of multipliers. For instance, if we want to convert inches to rods, we'll have to divide by 198 and if we want to convert inches to yards, we'll have to divide by 36 and so on.
the names in metric system tell you how to convert: meter, centimeter, millimeter.
inch, foot, yard, I need to lookup a dictionary to know the ratio to convertamong them.
All the units of the metric system are in powers of 10 so to convert, all you have to do is multiply or divide by 10 or just move the decimal. With the U.S. system, there is no consistency to the conversions.