To put it straight; It has to do with in the half lives in the Benzodiazepine class of meds. There are charts concerning the half lifes of the different "pams" Longer half lifes like diazepam and clonazepam are generally used for GAD with other co-morbid issues. The type like xanax are quick/ short acting and have a very fast half life usually for panic related issues. Study this in basic psychpharmo. classes at community colleges! Benzodiazepine Equivalency Chart: Benzodiazepine Equivalency benzo.org.uk : Benzodiazepine Equivalence Table