Why did the USSR preserve the national republics?

The Soviet Union, or Union of Soviet Socialist Republics, was an attempt by the Communists to recreate the Russian empire under "socialist" principles. The idea was to blur the differences in "nationalities" in favor of the common, socialist "good," so that the different peoples would forget their petty differences and work together for a worker's paradise. They did so to distinguish their government from the failed Tsarists.

The original "republics" were Russia, the Ukraine, Byelorussia, and "Transcaucasia," but the other "republics" demanded, and obtained their separate identities as a "consolation" for being absorbed by the larger USSR.

Just to elaborate on points already made, while the Soviets did preserve the national republics, Stalin actively took measures to limit their autonomy. A few significant examples of this are:

1) the gifting of Crimea to the Ukraine. Crimea was distinctly different in terms of its culture and history, and has no legitimate ties to being a subsidiary of Ukraine in terms of a historical perspective. 2) The rearranging of the boundaries of Kyrgyzstan and Uzbekistan, making them more convoluted and meant to maximize the conflict over the water system. 3) the enforced specialization of the Central Asian countries. Uzbekistan was turned into an area highly specialized in farming and agriculture, which made it logistically dependant on long term opponents the Kyrgyz who were in control of the water supply, and who were made into significant power manufacturers. Kazahkstan had many of its existing industries dismantled and specialized according to the needs of the USSR.

The purpose, as is theoriized retrospectively, was to maximize conflict in the case that the USSR should fall, in order to compel continuing cooperation between the Soviet nations and prevent true autonomy. The conflicts between Kyrgyzstan and Uzbekistan are a testament to this, as is the massive international conflict over Crimea today. The Central Asian states are essentially completely interdependent, and have not progressed significantly beyond their primary economic structures since the USSR.

The creation of up to 15 Soviet Republics was just a consequence of the decision made on 29 December 1922 to keep the 4 already existing republics. After that point, it would seem unjust and provoking not to add the next 11 it they served the same purpose as the original 4. (Also, there were many more autonomous republics.)

These four original republics of the Soviet Union (Russian, Ukrainian, Transcaucasian, Belorussian), were not created artificially by a central authority, they created themselves.

The Bolshevik coup d'etat would never have succeeded without people's support. It was the Czarist ("White") thing to fight for the empire, to subdue other nations, to rule them, to forbid them their own language or customs, to turn them into Russians (russification). This became extremely unpopular, and Bolsheviks (as well as virtually all anti-czarists establishments) boldly rejected all these practices; they would never have won if they hadn't. The Bolshevik party itself was without regard to nationality: people in the highest ranks apparently could be Georgians or Jews despite the politics happening mostly in Russian cities.

As local governments or armies emerged, no one was forbidden to speak local languages, so naturally internal borders appeared - the linguistic and geographical ones. They quickly caused the formation of separate republics.

This natural flow of events was later consciously supported by Lenin, who called it korenizatsiya (rooting-in).

At the end I have a rather long example showing the Soviet sensitivity to various nationalisms: the war of conquest. See, Red Army was not an imperialist conqueror. Seemingly there was no case, as long as the Soviet Union existed, when it did officially state anything like "every nation wants to expand as much as possible, therefore we take this land now for us". (Two times they were pretty close to saying that, but still kept a thin veil of some sort: Japan and Poland.) So how did the Soviet Union expand in all the directions? For example, Finland. Sarcasm warning. Did you think that Finland was invaded by Soviet Union in 1939? Wrong - that would be imperialist and thus unthinkable. Instead, the official line went that the Soviet Union firstly supported the Finnish politicians in creating their own socialist government (government in exile, made by the Fins for the Fins). Secondly, the new Finnish Soviet government urged the Finnish workers to liberate themselves from oppression of capitalists and imperialists. Thirdly, the Soviet Finnish government requested military help from all friendly nations (read: Soviet Union), but even then these nations hesitated to initiate hostilities, as it would be a form of aggression. Fourthly, the silly "White-Finnish" troops attacked a random innocent Soviet village. Ah, finally! Only then could the Red Army come (actually it was quite close and in a huge concentration, phew) and assist the new Soviet Finnish country in their struggle to liberate themselves. If the liberation succeeded, the stated reasons would inevitably cause the establishment of a Finnish Socialistic Soviet Republic, which would maybe decide by itself to join USSR.

The points to note here are the many precautions of the communists against putting themselves anywhere near the old Russian-centric imperialism; and to show they treat all nations equally. While in case of Finland all of this was staged, in earlier case of Ukraine all of this was true; most importantly, in all cases the stories were much emphasized by Soviet media.