Because a kilogram is still just "grams"; the relation between kilo-, milli-, etc. is linear. Metric units allow you translate, for example, 1 liter to milliliters in just one jump. And it's not like there are that many prefixes to remember.
The relationship isn't linear, it's logerithmic, just as the us volumetric measures are.
There are more steps in the power-of-two measures, but hardly anything unreasonable.
The real usability of the metric system, which I think you're getting at is that those prefixes are reüsable between all units, whereas in the us customary system it's no holds barred and everything outside the volumetric is arbitrary.
That said, with length, it is nice having units that break apart evenly at 2, 3, 4, and 6; but I don't think that's quite enough to redeem it.
