To add to this, before the advent of computer chips, we could not adjust timing "on the fly" to avoid detonation (gasoline igniting before the spark event).

For instance, the GM LS engine family has an "anti-knock" sensor that can detect the beginnings of detonation before its audible to the human ear, and it will then retard the spark to allow the cylinder in question to cool down a hair.

Before that technology, the compression ratio had to be chosen to work for all engines across the entire country, using all grades of fuel, even fuel with variable quality from a low-quality brand.

Now, the LS family of engines can come from the factory with a relatively high compression ratio.

In the early 1970's, lowering the compression ratio a bit was the fastest and cheapest way to lower cylinder temperatures, and thus create less emissions of the type that was being measured.