Because hating an outgroup is always easier than accepting that current hardship is the result of a complex interplay of factors that aren’t entirely understood and that might require counterintuitive solutions that aren’t guaranteed to work.
“Capitalism is good, therefore being rich is good” seems logically consistent, thus “too much concentration of wealth is bad for capitalist societies” is nontrivial and requires some mental work to understand. If the other guy says “nah, rich people are cool; it’s those other people that are different from you that run everything” it’s more immediately palatable.
So how do you get poor people to back the interests of rich people instead of their own? Segregate them and blame subsets of them. Don’t Be a Sucker is as relevant today as it was in 1947.