The short answer to your question is: it kinda just happened that way.

While early plans for post-war Germany called for a partial occupation of the nation, such as what happened after the First World War, the Allies eventually agreed to a full occupation. This occupation would serve two purposes. First, the Allies feared that the German government would cease to exist once its military forces collapsed. It was believed that many of the ministers would have been killed during the war, and the chaotic situation present in Germany would hinder any legal attempt at occupying Germany. Second, there was the idea that a full occupation of Germany would drive home the military defeat that the nation had suffered. One criticism of the Allies post-WWI was that, while they had defeated the German state, the citizens of Germany were not as inconvenienced as losing citizens often are. They were not witness to the horrible destruction of war as the German government surrendered before the Allies could push deep into German territory. This allowed for the myth of the 'stab in the back' to promulgate and was one of the reasons for Hitler's grasp of power. It was believed that by fully occupying the country, the German citizens would be taught a costly lesson which would make future conflicts avoidable. With these two points in mind, the Allies accepted the surrender of Germany's military forces and set up four separate areas around the nation which would be governed by the victorious Allied powers.

Now, the problem comes from the fact that following the Second World War, the Cold War broke out between the Soviet Union and the United States. Each of the nations had their own view of how Germany should be governed, and these views were colored by both previous experiences with Germany, and each nations geopolitical goals. Both Russia and France wanted to punish Germany for starting the Second World War and ensure that Germany could never again rise to become an economic juggernaut which could threaten the safety of its neighbors. To ensure the subjugation of Germany, Russia demanded war reparations while at the same time stealing industrial secrets and equipment from the Germans to rebuild Soviet industries. France demanded that Germany be split into separate economic zones and that its industries (except for the coal industry which France relied upon) be dismantled and never allowed to be rebuilt; and Great Britain and the United States wanted to bring Germany into a new liberal world order centered around trade and the free market. This new world order would be one controlled by the United States and Britain of course.

Long story short, the United States and Russia competed in Germany at the beginning of the Cold War. The United States was able to join their zone of control with Great Britain (and eventually France) and attempt to rebuild Germany's broken economy. The Soviet Union saw this as a threat to its influence in Eastern Europe, and an attempt by the West to rebuild an enemy which had invaded the Motherland twice in the past 30 years. Due to the disparate goals for post-war Germany that each Allied power had, and the misunderstandings ech of the powers had about the goals of the other powers, a schism formed between the German state, and an East and West Germany was founded. The policies enacted by each of the Allied powers during their occupation of Germany served to exacerbate the distrust of all the various actors in Germany and, instead of living up to the lofty ideals of peace and brotherhood which had emerged following the end of the Second World War, the Allied powers slipped into mistrust and suspicion.

So...Germany was originally split into four pieces. US and GB merged their pieces together, and eventually merged with France's smaller territory. Soviet Union didn't like this and built a wall.