They're standing by their man, even as he fails to live to his promises.
It's incomprehensible to many of us that people could support a president who, in Bernie Sanders' words, "is compulsively dishonest, who is a bully, who actively represents the interests of the billionaire class, who is anti-science, and who is trying to divide us up based on the color of our skin, our nation of origin, our religion, our gender, or our sexual orientation."
Based on various trusted sources and a dab of cognitive science, it's fair to conclude that there are three main reasons for this unlikely phenomenon.
1. Trump's Followers Believe They're Better Than Other People
Nationalism, exceptionalism, narcissism, racism. They're all part of the big picture, although it's unfair to simply dismiss Trump people as ignorant racists. Many of them are well-educated and wealthy. But well-to-do individuals tend to feel entitled, superior, uninterested in the people they consider beneath them, and less willing to support the needs of society. Thus many wealthy white Americans are just fine with Trump's disdain for the general population.
Poorer whites also feel superior, in the sense that they're reluctant to give up their long-time self-assigned position at the top of the racial hierarchy.
Trump and the Republicans don't seem to care at all about poor people, especially people of color. It's nearly beyond belief that they'd allow a father to be torn away from his family after living in the U.S. for 30 years; that they'd allow tens of thousands of Americans to sleep outside in subzero weather; or that they'd ignore the women and children being blown up by our bombs in Yemen.
2. They're Driven by Hatred for Their Perceived Enemies
According to an ancient proverb, the enemy of my enemy is my friend. For many besieged Americans, the friend is Donald Trump, the enemy of his followers' enemies, based on his belligerent put-downs of so many target groups that have been recklessly blamed for America's problems.