i'd hazard a guess that it was because the 440 wasn't available with a 2bbl option like a lot of the ferd and chebby stuff was.
even if the fuel mileage was the same, the perception of big carb on a big motor was immediately: gas guzzler; at basically, the worst possible time.
there also exists the probability that with encroaching emissions they weren't able to meet standards and instead of flat out castrating the chonk block and turn it into the a mediocre offering they just decided to phase it out.