Spark lead got less in 1972 because it was 1972. Starting in 1970 the government forced emmision restriction on the big 3. Smog in California.Smog was caused by nitrogen combining with oxygen in the cubustion chambers of every vehicle. This byproduct grows as heat and the pressure increases. But, heat and the pressure it creates is the only thing that pushes on the piston. The only thing that makes power is piston push. So, to get rid of nitrous emmisions combustion temps needed to drop. Read as pressure. Read that as power.
This worked to drop nitrous emissions. Stomp on it: no pressure or power = no fun and no smog.
But, the low compression motors were so crappy at burning the fuel, because of the government mandated artificially low partial throttle cylinder pressures, that a new problem showed up. Hydrocarbon emissions (which is a fancy name for gasoline). The un-burnt mixture was getting emitted now.
Solve that by putting a self heating chamber in the exhaust that self heats by catalytic reactions. This heat forces the unused fuel and oxygen to burn in the exhaust (where it does no good). They call it a catalytic converter.
Now, you've got water vapor and co2. Lately, I've been hearing rumblings that co2 isn't good for the planet either.
Anyhow, lead from the gas would coat the catalyzing surfs in the converters and render them useless. They couldn't have that, especially now that your new car is at a 8:1 compression ratio and you don't need that octane anyhow.