WWII was much easier to sell to the people, because the nazis were invading _now_. It seems impossible to sell reduced standard of living today to respond to a problem tomorrow
Even with the obvious danger the Nazis presented it took a long time for most countries to respond. Britain and France could have stopped them (from a much stronger position) earlier but did nothing until after they had conquered Austria, Czechoslovakia, and Poland) and very little until the Nazis actually invaded France. The US did not enter the war until it was attacked either. Nor did the Soviet Union.
There was reluctance to act on an obvious and imminent threat. People just hoped it would go away.
Nazis were invading other places now. The US cut off Japan's oil supply, triggering an existential crisis for Japan that led to a military attack.
If United States had joined the Axis powers, leadership very plausibly could have continued the practice of isolationism (which was the popular opinion before Pearl Harbor). It might have been a shortsighted move, but Nazis invading Europe did not sell the US population on entering the war.
There was reluctance to act on an obvious and imminent threat. People just hoped it would go away.