My 'guess' is because of many situations: Emissions testing/certification, too many engines offered, and lastly because of the competition from other manufacturers. Remember, Chevrolet had the (V-8s) 302, 307, 350, 396 and 427s. Ford had the 302, Boss 302, 351, 390, 428, and Boss 429. Mopar: 318, 340, 383, 426 Hemi, and 440. Also, include all of the other GM divisions engines, and you get the idea. All were pretty close in CID and HP ratings among each manufacturer for sales dominance.