The short version is that the Federal government started requiring catalytic converters be warrantied for 120,000 (I think) miles. As the phosphorus component of ZDDP can destroy the cats, vehicle manufacturers started looking to reduce the chances of phosphorus getting into them. So they started lowering the limit allowed in oil to levels that are good enough for modern engines. Since flat tappet, overhead valve, high valve spring pressure engines are essentially a thing of the past, oil reformulated with anti-wear components other than ZDDP work just fine.