The quick answer is that the Democratic Party isn’t socialist. Socialists work against the interests of capitalists and guess who the Democratic Party takes a lot of money from? The few socialists or democratic socialists that try to run through the Democratic party are fighting an uphill battle and are only doing so mostly because the two party system makes it impossible for 3rd parties to win in most cases.

This has always been the case, but what might cause this confusion is that the Democrats appeared to favor more socially oriented policies in the mid 20th century with The New Deal and The Great Society. But the thing to understand about that is:

• Despite creating some social spending programs, they kept capitalists in power.
• They never stopped doing the other part of capitalism: Imperialism.
• There was a lot of pressure from outside the government. Unions were stronger. The Great Depression was the greatest crisis capitalism had seen up until that point, and the success of communist revolutions in other countries could have shown the American working class a different path forward.

In the 90s, with the Soviet Union dissolved and the power of unions thoroughly gutted, the Democrats under Clinton did a realignment to the right. Clinton famously passed welfare “reform” (read: gutting it) calling it “an end to welfare as we know it.” Clinton entered us into NAFTA, a trade deal that helped facilitate corporations moving production to other countries to exploit cheaper labor. He passed the Crime Bill which is credited with being a huge contributor to mass incarceration. Etc.

Since then Democrats have looked a lot more like Clinton than FDR, and even FDR wasn’t a socialist. So yeah, the people who helped take things away from the working class aren’t super thrilled about someone who wants to take some of that stuff back for us.