Because the Soviet Union was badly bleeding and Stalin could not be completely confident it would not bleed out and collapse.

By the end of 1943, the Soviets had suffered 8.7 million irrecoverable military casualties (according to G. F. Krivosheev). This was a huge number that caused significant issues for the Soviet military. German losses were also severe, with 1.7 million lost by the end of 1943. However, the Germans still had 176 divisions on the Eastern Front and were still fighting very hard. Some Soviet offensives had been successful, but some 1943 offensives had failed and incurred disproportionate casualties on the Soviets.

There was absolutely no guarantee that the eastern front wouldn't turn into a bloody stalemate, and no guarantee that the Soviets would not bleed out before the Germans did.

But in the end, of course, that's not what happened. 1944 was the year in which the Nazi forces were shattered beyond recovery. What caused this? Firstly, the Western Allies air offensive in the early spring of 1944 broke the German Luftwaffe. From this point on the Luftwaffe was not a factor in blunting Soviet attacks as it had been at times in 1943. Secondly, the Allied landings in France in June of 1944 pinned down (and eventually destroyed) the Axis reserves, with approximately 105 divisions committed to Italy, Norway or the West. Then in July 1944, the Soviets launched their Bagration offensive which reduced the German forces on the eastern front from 176 divisions down to approximately 130 by September. These three events in 1944 are what cracked the German military and made their defeat obvious and inevitable.

After that, the Soviets had achieved overwhelming superiority, but the Western allies were already at the border of Germany after having pursued the German armies across France and inflicted heavy losses. Now jockeying for postwar strategic advantage could begin in earnest, but it would have been premature before the end of 1944.