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 forbaddin 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. I believe in no case as long as it existed did the Soviet Union officially state anything like "every nation wants to expand as much as possible, therefore we take this land now". (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, take Finland. 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). 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 the nations hesitated to start 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 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, who 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.