THis comes from practical and historical considerations. Mass is probably most commonly used quantity by non-technical people, so it is desirable to maintain co

THis comes from practical and historical considerations. Mass is probably most commonly used quantity by non-technical people, so it is desirable to maintain continuity with previous systems which had gram as base unit. On the other hand kilogram as base unit makes many useful relations between various quantities simpler not to mention that it is significantly more practical to make and maintain prototype of kilogram, than prototype of gram.