I was going to say something similar. Didn't it have more to do with the automakers simply didn't know how to meet emissions requirements and were learning as they went ? You had things like a '78 Corvette that had a whopping 150hp. Or, depending on your age, remember Magnum P.I.'s Ferrari 308 ? It looks great - I still love the look of that car - but had a little over 200hp.
Yep, they were were forced to learn how to comply, and there were different approaches, none of which worked well in those early days.
It could be disheartening to see models in some markets have "full" power (yet still low), while those in the clean markets rated for even less, due to the controls. That played out on a country by country level, and domestically, by state, before there were "50 state" models.