Do you mean why in vivo preclinical studies prefer to use weight/body weight
over molarity?
The simple answer is that when it converts to human dose for later clinical
study, mg/kg is a lot easier.
Also, for PK/PD study, body weight adjustment is a critical factor to
consider, mg/kg dosage gives a simple/fast way for drug administration on
different mice.
uM (as concentration) does not give specific information on the drug dose,
so you have to specify the volume too.
The last but probably the most important one is that nurses usually don't
have much idea about molarity at all......