Ruthenia was ethnically Ukrainian. Ukraine was at the time a constituent republic of the Soviet Union, so they were joined. This was the same justification used for annexing large swaths of eastern pre-War Poland to the Belarusian and Ukrainian SSRs, as those areas were largely ethnically Ukrainian and Belarusian

The overarching justification was that Stalin and much of the Soviet leadership had come to the conclusion that the basis of the postwar order in Eastern Europe would be territorial-based ethnic homogeneity. This meant that a large scale population transfers, often by force, especially the remaining Volksdeutsche populations, in areas that the Soviets deemed vital for future national security. The underlying strategic rationale was that such homogenous populations would result in stable polities that would be reliable satellite states. The strong anti-German sentiment also reenforced this belief as Soviet held two mutually compatible views. First, the entire ethnic German population was directly complicit in the Third Reich's genocidal war against the Soviet population and thus deserved punishment. Second, pushing Volksdeutsche population westwards would eliminate any fascist revanchist movement. Now neither of these suppositions had too much of a basis in reality, but they were operative in the minds of Soviet policymakers.

Of course, this restructuring of borders and populations would be done in a fashion that benefited Soviet power and the USSR. In the case of Carpathian Ruthenia, the Soviets' public stance was that the Rusyns were an ethnic subgroup of Ukrainians that were now being reunited with their own national state, the Ukrainian SSR. Now whether or not Rusyns are Ukrainian is a highly thorny issue predicated upon a complicated reading of linguistics and culture, but the Soviet position did not really take a very nuanced approach in their definitions of ethnicity and often projected an ethnic homogeneity onto regions that was often quite fictitious. Borderland regions are often characterized by a very plastic form of identity that does not lend itself well to the concrete definition that the Soviets applied to nationalities. This disjuncture between reality and ideology led the Soviet state to a series of harsh measures to Ukrainize the Rusyns. There was an attempt to replace the Ruthenian with Ukrainian language in the schools and the state forcibly incorporated the Ruthenian church into the Russian Orthodox church.

Sources

Snyder, Timothy. The Reconstruction of Nations Poland, Ukraine, Lithuania, Belarus, 1569-1999. New Haven: Yale University Press, 2003.

Wolff, Larry. The Idea of Galicia: History and Fantasy in Habsburg Political Culture. Stanford, Calif: Stanford University Press, 2010.