Americans classically derive much of their sense of value from:
1. Work
2. Race (for many white people, especially historically)
...and that's about it. Unlike other cultures our "rugged individualism" means that we don't tend to derive as much sense of worth from family, traditions, organizations we belong to, etc. This is especially true for men who are raised to believe that who they are is what they do.
The economic shift to a service- and knowledge-based economy with many manufacturing and ag jobs being automated or outsourced away has devastated 1. in many communities. The march towards better civil rights and equality is chipping away at 2.
This has left a huge number of Americans feeling that they are worth less than they used to be. People like that are ripe for being exploited. People will buy anything if you tell them you're selling dignity. Trump's "Make America Great Again" slogan was really "Make You Feel Like You're Great Again".