[king_midas]

I've heard it was because of a couple of reasons, some mechanical, and some reputational.

Mechanically, because of the use of the corp. 10-bolt axle as a perceived weak-link.

Reputationally, because performance was increasingly becoming an albatross around the neck, and the mfgr's knew the writing was on the wall regarding big engines, fuel, mpg, etc... In other words, there was no good reason to continue the engineering and federalizing of those engines into those cars, so they didn't do it.