"[...] specifically monitoring performance in six particular categories: gross domestic product per capita, social support, healthy life expectancy, freedom to

"[...] specifically monitoring performance in six particular categories: gross domestic product per capita, social support, healthy life expectancy, freedom to make your own life choices, generosity of the general population, and perceptions of internal and external corruption levels." US ranks pretty high in GDP-per-capita. Given the remaining factors, I imagine the obesity rate and intensified political polarization is skewing the result. The perceptions might be informed in large part by rate of consumption of news media content on social media (other developed countries have this problem, but lesser polarization may mute the impact). Among good news is that the obesity rate has stalled in recent years, iirc.