It's so that it retains coherence without moving to a non-gram based measurement. You could have easily selected a new unit, or even just given what we call a

It's so that it retains coherence without moving to a non-gram based measurement. You could have easily selected a new unit, or even just given what we call a "kilogram" a non-prefixed name and be done with it. The point is, it's obvious that they didn't define the system on first principles, but allowed "creature comforts" to creep into the scope of the system. For example the needless definition of Hz which conflicts with radians, and the needless definition of the unitless mol. These are obvious conveniences. They're not first-class members of the system.