Vibrant pigments usually rely on heavy metals. Lead was common in Red and green paints, arsenic was in greens and chromium was in blue. When they were replaced with organic pigments. They either lost their vibrancy or degraded and faded very quickly.

Also, paint is spread very thinly on cars compared to in the old days. This is partially due to higher quality steel, partially due to more plastic panels and mainly due to weight and cost reduction. This means that doing a cut-and-polish to liven up faded paintwork is liable to expose undercoat.