Emissions, product liability (remember the GM saddle tank fiasco?) and lack of demand would be my guess.
The funny part is the saddle tank fiasco was a media-contrived fraud. Never was a real problem. Yet I've got an unused rebate letter / check laying around here because of it. IIRC it was $2k off a new vehicle, and something much less if I transferred it to someone else. Still got my '85 with twin 16gal 'saddle' tanks.