I remember looking at a post in Passenger Car Oils, and one poster guessed that since Pennzoil is one of the most common oils used at oil change centers and auto repair shops, that it increases the odds that someone who doesn't change their oil enough will use it.
The other theory was that because Pennzoil was common, and in the 1970s, badly designed smog devices and PCV systems filled engines with abnormally high sludge, PZ took the blame, when in fact, unreliable smog devices pushed the oil well beyond what it was meant to do.