As lots of things in history, there was more then one reason:

-The Russians had a huge army, they suffered heavy losses, but they still had a huge army.

-The allies were not willing to pay the price to fight the USSR in both man power and equipment.

-The war with Japan was still raging and the allies wanted the USSR cooperation, remember the trivia about no purple star being made ever since the preparation to invade Japan.

-The weather, Russian winter had just played a vital role on stoping the German army, no one wanted to make the same mistake a couple of years later.

-The scale of the thing, from Berlin the allies would had to push the Russians back close to 1000 miles (1600Km) to get to Moscow in a front spanning from the north sea to the balcans and south of Europe, that would alone would require millions of men and years of fighting, and the Russians had already proven that they could retreat another 1000 miles from Moscow.

-The price on the civilians the allies had just started to shake the German people from their devotion to Hitler, they wanted to rebuild to keep them on their side no destroy whatever was left standing at that point, this is true for the rest of Europe without the devotion to Hitler (in most cases), the continent was destroyed.

-The division of Europe had already been agreed upon the Ialta, Tehran and other conferences, so the allies could protest, but they really couldn't/wouldn't do anything about it.

Of course the Americans eventually had the atomic bomb, but it was more complicated to drop a bomb no the USSR than in Japan, the USSR main cities were far away and bombing one or two would not make them surrender, and they were too far away, the USA bombers would have to fly thousands of miles over the USSR and they didn't have the air superiority they had in Japan, they would have been shot down for sure.