Explain why many Americans in 30's supported a policy of isolationism .

Many Americans in the 1930s supported a policy of
isolationism because they did not want the US to be pulled into another war in the way
that the country had (they felt) been pulled into World War
I.

Many Americans felt that WWI had really not been any of
America's business.  They felt that the country had been pulled into the war because of
the demands of businesses that had trade ties with the Allied Powers in Europe.  Because
of this, they wanted policies that would avoid this sort of problem happening again. 
They got their wish when Congress passed the Neutrality Acts of the
'30s.

Overall, then, Americans during this time supported
isolationism because they did not want to be drawn into more destructive wars that were
not really any of the US's business.