Introduction

The answer to this question, put concisely, is that at its inception the Soviet Union tried to style itself as a sort of "post-nation-state" nation-state but was forced to behave more and more like a traditional international actor as time progressed. As such, simply annexing enormous swathes of territory instead of organizing puppet regimes was much less realistic by the time the world had just finished ridding itself of a man who had tried to do just that in Adolf Hitler. The Warsaw Pact was meant to act as a loyalist buffer to the North Atlantic Treaty Organization (NATO); Stalin had just learned his lesson concerning annexing territories all the way up to your archenemy's border the very, very hard way. Also keep in mind that the Soviet Union was not a contiguous state the way France or Germany is, it was subdivided into 15 "independent" Soviet Socialist Republics (SSRs). This map shows the Soviet Union as a collection of smaller states.

At its inception in 1922, the Union of Soviet Socialist Republics (USSR) was designed to be a continually growing organism that simply added new "liberated" territory to itself as the Bolsheviks were able to orchestrate revolution further and further from the epicenter in Saint-Petersburg, Russia. Before the Soviet Union was created, after the Russian Civil War and all of its accompanying wars of conquest came to a close, there was an intense debate about how to organize and consolidate the gains made by the Red Army. Essentially you had Lenin's Revolutionary Vanguard with control over huge swathes of territory which used to be a part of the Russian Empire, though they were never considered "Russia," even during their subjugation, but they had no real pragmatic way to manage them. I'm talking about places like Kazakhstan, Mongolia, Georgia, Azerbaijan, Turkestan, and the Caucasus region (which itself contains several small countries like Chechnya, Dagestan, and Ingushetia). This map provides a really nice illustration of the Russian Empire before World War I (the resolution of which required the Bolsheviks to concede an enormous amount of territory in order to extirpate themselves from the carnage-- this territory was just out and out liberated after Germany's defeat, but very importantly, not reintegrated or re-associated with Russia in any way: primarily the Baltic States, Poland-- which included parts of Belarus and the Ukraine, as well as Finland).

As the Soviet Union came into being, they styled themselves as a new nation state which only existed on Russian territory by happenstance. The Bolsheviks initially intended to be Internationalists, or post-national. This was a romantic position to take of course, but they largely had to back track on it as the practical realities of international politics dawned on them, and getting things like military and financial aid became increasingly necessary.

The Theoretical Problems of Empire

See there is a philosophical problem the Bolsheviks were facing here: a bourgeoisie imperialist society fundamentally believes in the superiority of its own culture and is therefore thus able to justify the subjugation of its colonies, but a burgeoning socialist republic is supposed to supersede constructs such as "nationality" and "nation," so how do you justify controlling territory that doesn't inherently "belong to you?" Ergo, the Bolsheviks had to find a way to enforce the same sort of de facto state as a traditional empire without using the same justifications. So obviously, you now have to answer the classically unanswerable question of how many angels can dance on the head of pin? The Bolsheviks labeled this discussion The National Question, and its resolution is extremely important to the answer to your question.

The Soviet organ which was most responsible for handling these types of issues was the People's Commissariat for Nationalities, which basically acted to reconcile the concerns of minority populations throughout the USSR. The first head of this department (appointed in 1917) was none other than Ioseb Besarionis dze Jughashvili, better known to the toiling masses as Joseph Stalin. Stalin was born and raised in the north Caucasus region in what is today Georgia, so his appointment made quite a lot of sense: he had all the requisite Marxist bona fides and was outwardly unflinchingly loyal to the father of the revolution Vladimir Lenin. However, The National Question was one area where Lenin and Stalin never quite saw eye to eye.

Consider Lenin's writing from his Theses on the National Question (1913):

"Social-Democracy... must give most emphatic warning to the proletariat and other working people of all nationalities against direct deception by the nationalistic slogans of “their own” bourgeoisie, who with their saccharine or fiery speeches about “our native land” try to divide the proletariat and divert its attention from their bourgeois intrigues while they enter into an economic and political alliance with the bourgeoisie of other nations and with the tsarist monarchy." (italics in the original, text of the essay)

That essay is rife with similar sentiments, but this excerpt very nicely underlines Lenin's essential position. The "nation" and the "identity" is a bourgeoisie tool to divide the proletariat and ought to be ignored, or better yet, eradicated. Therefore, the USSR should just be one big nation-- not a collection of tightly affiliated but nominally independent entities.

Now consider Stalin's writing from Marxism and the National Question (1913):

"When the workers are organized according to nationality they isolate themselves within their national shells, fenced off from each other by organizational barriers. The stress is laid not on what is common to the workers but on what distinguishes them from each other. In this type of organization the worker is primarily a member of his nation: a Jew, a Pole, and so on. It is not surprising that national federalism in organization inculcates in the workers a spirit of national seclusion.

Therefore, the national type of organization is a school of national narrow-mindedness and stagnation." (italics in the original, text of the essay)

It sure sounds like the two agree doesn't it? Stalin makes a very subtle, but he claims, very important distinction in the first section of the aforementioned essay: while Lenin (and many other Marxists-- including Marx himself) often used the words "nation" and "state" interchangeably, Stalin argued that the two were inherently different. In short he says a "state" is just lines on a map-- like the Russian Empire, or the Austro-Hungarian Empire, or the British Empire, etc. but that a "nation" has a specific set of characteristics which he outlines in the first section of that essay linked above called The Nation:

• A common language
• A common territory
• A common economic life
• A common psychological make up

Thus, Stalin intones:

"A nation is a historically constituted, stable community of people, formed on the basis of a common language, territory, economic life, and psychological make-up manifested in a common culture." (italics in the original)

As such, Georgia is a nation, Kazakhstan is a nation, Russia is a nation, but the Soviet Union is a state. Stalin's answer to The National Question is much more akin to modern Marxist political thinkers: each "nation" must experience the revolution in a way completely unique to that community. Think Socialism With Chinese Characteristics, Titoism (Socialist But Independent), Juche (North Korea), etc.

Considering Stalin and Lenin were having this intellectual debate as far back as 1913, and Lenin appointed Stalin as Commissar of the aforementioned Commissariat of Nationalities in 1917, it seems that Stalin's idea was accepted by him and other Marxists as valid political thinking.

This is the theoretical answer to your question: because a nation is technically indivisible, the state of the USSR must be constituted of distinct nations.