Answer:
Because he believed that the success of capitalism would force the Soviet Union to change.
Step-by-step explanation:
During the Cold War the United States and Union entered into a conflict and disputed the influence in other countries. As the United States tried to expand the capitalist system among other countries, the Soviet Union tried to do the same with the communist system.
Truman believed that capitalism had a much greater chance of expanding into the world and creating a sovereign force to the point of forcing the Soviet Union to change its political vision. This is what Truman means in the excerpt above.