These issues are yet again the problem of our age. Their seeming trajectory towards resolution post WWII, with widespread prosperity and a rising middle class, has been undone. What undid them points to the underlying problem: Immediate causes include the spectacular increase in financialization and unearned rents, the lack of and lack of enforcement of progressive taxes, both in turn largely due to a shift in the public's understanding of these issues. What caused this shift in public understanding is the age old problem—power and the lack thereof.
Inequality
The sentence in Ben Shapiro's book preceding the title quote is this: "Discussions of income inequality, after all, aren't about prosperity but about petty spite."
I'm sorry Mr. Shapiro, but this is entirely about prosperity. And not at all petty.