Either too old for that particular wine, or stored improperly (too hot), or just not a very good wine to begin with. "Cru bourgeois" denotes an everyday drinking wine that may not necessarily improve with age.
Of course, many high quality 1996 bordeaux are still too young to drink or are just now entering their prime drinking window, but this does not appear to be the case with your bottle.
Do not be discouraged. The rewards of finding a good 10+ year old bordeaux that has aged well, especially at a modest price, more than make up for the occasional disappointment.