Why do you think President Truman believed that poverty and suffering created conditions in which communism might spread?

Communism was based on a defence of Egalitarianism and thus poor people could resort to it as an alternative. The ravages of the war were seen as a major cause of the spread of communism in Eastern Europe notably.

The social unrest due to the extreme poverty, unfulfilled hopes, and world war I lead to the take over of Russia by the communists.

The economic depression in Germany allowed the socialistic Nazi's to take over Germany,

The social disruption after world war II left Europe vulnerable to revolutions. The Soviet Union had occupied and created communistic governments in most of Eastern Europe. Extreme poverty could create social unrest that the communists could exploit.

The Marshall Plan provided grant of money that could be used to rebuild the economies of Europe. The Marshall Plan moneys were offered to the Eastern European Countries not just to the Western European Countries. The Soviet Union forbade the Eastern European countries under their control from accepting the money.

The Marshall plan was effective in preventing social unrest in Europe. Truman feared that poverty and the resulting social unrest could result in the spread of communism.