That depends which view you take. The official reason is that they were expanding forces and waiting until they had the highest chance of success. It is also suggested that they waited that long in order for the German army to be weakened by the USSR and so the USSR sustained higher casualties, thus weakening them post-war.
Stalin believed that the delay in opening a second front in Europe during World War II was primarily due to the Western Allies' reluctance to engage fully against Nazi Germany. He suspected that the Allies, particularly the United States and Britain, were intentionally prolonging the conflict to weaken the Soviet Union and reduce its influence in post-war Europe. This perception fueled distrust between the Soviet Union and the Western powers and highlighted the differing strategic priorities among the Allies.
they hoped that Germany and the Soviet Union would cripple each other
Teheran
While the Allies could have eventually defeated Germany without a second front in Europe, it would have likely taken significantly longer and resulted in higher casualties. The opening of the second front in 1944, through the D-Day invasion, was crucial in diverting German resources and attention, accelerating the collapse of Nazi Germany. Without it, the Soviet Union would have borne more of the burden in the east, potentially leading to a different post-war balance of power in Europe. Ultimately, the second front played a vital role in hastening the end of the war in Europe.
D-Day in June 6,1944.
Stalin was upset with the Western Allies' decision to focus on North Africa during World War II because he believed it delayed the opening of a second front in Europe, which he felt was crucial to relieve pressure on the Soviet Union fighting against Nazi Germany. He feared that by prioritizing North Africa, the Allies were allowing German forces to regroup and strengthen their position on the Eastern Front. Stalin's frustration was compounded by the heavy toll the Soviet Union was suffering in terms of casualties and resources. He wanted immediate action to support the Soviet efforts and weaken German forces directly in Europe.
Josef Stalin, leader of the Soviet Union
Rome and allies versus Carthage, and the Western Europe people at the centre of the contest.
A significant cause of Soviet mistrust of the Western Allies during World War II was the West's delayed opening of a second front in Europe, which the USSR believed was a strategic maneuver to weaken Soviet forces. Additionally, the ideological differences between the communist Soviet Union and the capitalist Western nations fueled suspicion, as did the Western Allies' previous non-aggression pact with Nazi Germany. Incidents such as the Western reluctance to share military technology and intelligence further exacerbated these tensions, leading to a deep-seated distrust that would persist even after the war.
The Axis Powers in Europe formally surrendered to the Allied Forces.
By opening a second front the Allies would be able to draw off German strength from the Eastern Front and help relieve pressure upon the Russians .
The allies were desperate as the German force threatened to overwhelm all of Europe. Additionally, the Soviets needed the creation of a "Second Front" to relieve pressure on the eastern front. Without a successful operation, the allies may not have been able to get all the necessary supplies and troops into Mainland Europe.