Can you clarify as to why you think they would have conquered all of Europe?

The Yalta Conference of February 1945 laid the foundation for the Soviet empire that would last through the Cold War. This was a major meeting of the Allied powers that included FDR, Stalin, and Churchill. At this conference, Stalin demanded that the Soviet Union be allowed to create a Soviet "sphere of influence" that could act as a buffer region between the Soviet Union and a potentially resurgent Germany some day in the future. In particular, Stalin wanted to prevent the Polish government-in-exile, stationed in London, from acting as the new Polish government. Instead, he wanted the Soviet-installed government (the Soviet Union had already made it to parts of Poland by this time) to oversee the transition of Poland from wartime occupation to democratic elections that would take place after the war. Because FDR wanted Stalin's help both in defeating Japan (the atomic bomb was not a sure thing at this time) and, more importantly, in maintaining the United Nations after the war, he conceded that the Soviet Union could create this sphere of influence in the areas it had liberated, though FDR and Churchill both still hoped that Stalin would eventually allow for free elections in this region.

This summary of the Yalta Conference is, I think, very important in understanding the answer to your question because at the end of the Second World War there was no possibility that the Soviet Union would start a new war with the United States to extend its dominance further west. After all, the alliance with the United States and the United Kingdom still stood, the Red Army had no chance of defeating the other Allied forces (the scale of Soviet causalities went far beyond anything that the Americans had so far suffered), and he even sought their concession in order to build his sphere of influence in the lands that he had already liberated. Distrust grew during this time (the foundations of the Cold War can be found in his period), however, enough so that Stalin diverted forces directed at Berlin in order to ensure that the Red Army would liberate Prague and incorporate it into the Soviet sphere before the American army, which had stopped 50 miles west of Prague in order to allow the Red Army a full opportunity to get there first (it turns out that Truman, now president, how no intention of breaking the deal FDR had made at Yalta). There was distrust now, sure, but this only underscores the point that Stalin knew that he could not directly challenge American power in Europe at the time -- he feared that the American army could undermine his goals simply by liberating regions that he intended to influence.

Once the war was over and the Soviet-American alliance gave way to the Cold War, a potential Soviet invasion of Western Europe became a central fear that guided American foreign policy for the next few decades. This is why the United States stationed both troops and nuclear weapons in Western Europe. Had the Soviet Union invaded Western Europe anytime after the Second World War, it would have faced off against American troops in the region and spawned nuclear war globally (and until 1949, the United States had a nuclear monopoly). The Soviets did try to push the West out of West Berlin (an enclave within East Germany) with the Berlin Blockade, but this failed as the Americans countered with the Berlin Airlift. Fear of a Soviet invasion also spurred the creation of the North Atlantic Treaty Organization, which in turn compelled the Soviet Union to create the Warsaw Pact Bloc. An "Iron Curtain" (as Churchill called it) descended on Europe that divided Europe into two powerful camps, neither of which could attack the other without risking the absolute annihilation of Europe. Neither the United States nor the Soviet Union--despite rhetoric from both sides hinting otherwise--would ever challenge this status quo during the Cold War (and there are some very interesting examples of their continued tacit recognition of the agreements reached at Yalta).

As a side note, by the late 1960s, the United States had become so assured of Soviet pacifism in the region that the American Congress even considered withdrawing American troops from Western Europe. The Johnson administration opposed this and the Soviet invasion of its ally Czechoslovakia in 1968 (to put a stop to the Communist reform movement now known as the Prague Spring) provided the administration with the opportunity to end this discussion.

I fear this might be a loaded question. Asking why the USSR did not stay mobilized to conquer the rest of Europe after Germany surrendered is like asking why Britain didn't do the same thing -- it was never their aim to do so. It goes without saying that WW2 devastated Europe, and perhaps the USSR more than any other country, and all parties involved sought nothing more than a conclusion to the conflict. Even Japan, the last country to surrender, knew the end was near and were only holding out for a chance at better negotiating terms until the atomic bombs dashed those hopes.

Nevertheless, even if we pretend the Soviets were entertaining the idea of a European invasion after the war, here are a couple reasons that this just wasn't feasible.

Resources/war fatigue: The USSR suffered around 27 million (combined military and civilian) deaths during the war; far more than any other nation, and roughly 14% of the nation's total population. Furthermore, their economy was devastated with about 25% of their capital resources completely destroyed and their economic output in 1945 was considerably lower than prewar levels. It took everything they had to survive the Battle of Stalingrad and ultimately defeat a weakened German army by 1945. While no easy task, it suddenly looks a lot easier when compared to the potential task of fighting off the other Allied Powers. This leads to reason number two...

Resistance: the alliance between USA, UK and Russia basically existed only because they had a common enemy in the Axis powers. Beyond that they did not trust each other very much. A perfect example of this is atomic research. While the Russians began their own efforts to develop an atomic bomb by 1943, they were well behind the United States who deliberately did not share their knowledge with the USSR. And while not a primary or even secondary reason for dropping the bombs on Hiroshima and Nagasaki, some argue that a partial motivation for doing so was a show of force to the Soviet Union in case they had designs on conquering Japan and dividing it up for their own purposes. By time time the USSR successfully tested their first atomic weapon in 1949 NATO had already been formed (more on that in a bit), and within a decade both England and France had atomic weapons of their own along with the U.S. developing the hydrogen bomb. I don't think we need to dive any further into the role of nuclear deterrence preventing another all-out war in Europe, so let's go ahead and take that for granted while we move on to the final reason the USSR did want or need to invade Europe.

A better path: While the USSR did not aim to conquer Europe, they did have motivation to establish a "buffer zone" to prevent themselves getting invaded again like they had in WW1 and 2. This was further exacerbated by the formation of NATO which established an alliance between the US and much of Western Europe and was expressly created to combat the threat of a potential Soviet attack. This in turn led to the Warsaw Pact in 1955, which was basically an Eastern European NATO spearheaded by the Soviets. Under this arrangement the Soviet Union was able to exert ideological (and at times military) pressure over the Warsaw Pact satellite nations without needing to invade/annex them, which was more in line with their goals anyway (promoting communist revolutions across the globe, rather than installing communist governments by force). In short, the USSR was able to keep much of central and Eastern Europe under its thumb without waging war and got their buffer zone in case anyone tried to invade them again.

TL;DR: the Soviet Union was like just about everyone else in that they just wanted WW2 to end so they could rebuild. They also already had to worry about getting nuked by the United States, and by the time they had nukes of their own there was already a new military alliance (NATO) ready to deal with the USSR if they tried anything.

Sources:

www.shsu.edu/~his_ncp/Soviet2.html (EDIT: this isn't formatting correctly but there should be an underscore between "his" and "ncp" in the link)

www.atomicheritage.org/history/soviet-atomic-program-1946

www.icanw.org/the-facts/the-nuclear-age/

history.state.gov/milestones/1953-1960/warsaw-treaty

en.wikipedia.org/wiki/World_War_II_casualties (I know Wikipedia is frowned upon here as a source but they have good data and tables on WW2 casualties)

This was a real worry for Churchill in 1945. In fact, plans were drawn up for a counter offensive if the USSR didn't stop in Berlin and these plans also included the use of what was left of the German army by the allies against Stalin.