> Comparing the two scenarios, we found that about half of the observed decline in US social trust may stem from: i) ever more unemployment experiences, ii) ever less confidence in political institutions, and iii) a slight but systematic decrease in satisfaction with income.
https://www.sciencedirect.com/science/article/pii/S0049089X2...
I would also wager it has to do with increasing social and economic stratification and decreasing homogeneity.