Original: When the biological significance of nicotinic acid was realized, it was thought appropriate to choose a name to dissociate it from nicotine, in order to avoid the perception that vitamins or niacin-rich food contains nicotine, or that cigarettes contain vitamins.
Simplified: When the properties of nicotinic acid were discovered, it was thought prudent to choose a name to dissociate it from nicotine, in order to avoid the perception that vitamins or niacin-rich food contain nicotine.