When the General designed the early Chevelles, and even the later ones, they weren’t building track cars. Instead, their goal was to churn out basic, affordable transportation for the masses. And, even when they did stuff in some performance application, it was typically confined to driveline upgrades. When it came to the suspension, their goals were low cost, ease of manufacturing, and durability as it related to the warranty. They weren’t building uber-strong race car parts since most people buying Chevelles were picking up groceries and not time slips.
Given those facts of life, GM equipped their A-bodies with stamped steel arms that fell into the “strong enough” category with pinion angles that were in the general ballpark. Add in the fact that well over 40 years have passed and not only do we have better technology today, but those factory installed parts are really starting to show their age.