Answer: Because of the possibility of Germany winning the war.
Step-by-step explanation:
The United States entered the war in 1917. Germany had a great advantage on the European battlefield, so the United States' entry changed the balance of power. Economic factors are one of the key elements of entering the war. Namely, the United States sold large quantities of weapons to Britain and France. There was a possibility that those countries would lose the war, and in that case, they would not be able to repay the debt.
The situation in Russia was complicated. The country did not have developed industry and was not ready for war. The largest number of victims during the war was among the Russians. The country also faced a revolution, the king was overthrown, and the Bolsheviks took overpower.