a few reasons
1. NAC in pills can easily oxidize and therefore be rendered totally useless no matter what form it's in.
2. supplements in the United States are unregulated. so a company can claim that they manufactur the NAC so it's not exposed to air, but there's no way of telling. On the other hand, NAC from Canada or Europe is regulated by the government (much like our Rx-es here in the US). So BioAdvantex PharmaNAC from Canada is used by Dr. Moss from Stanford who is conducting Phase I, II and now III studies for the CF Foundation.