There is actually a good discussion of Soviet/Eastern bloc's love-affair with rebar here: Can anyone provide a historical context for Soviet urban planning?.

The long and the short of it is that even though slab concrete buildings were never a terribly popular among the public and films like The Irony of Fate satirized their lack of aesthetic charms, this architectural style did alleviate the postwar housing shortage. And believe it or not, at the time the USSR began to use re-bar extensively (the 1950s and 60s) Brutalism was a cutting edge and modern architectural style in the West. A number of architects celebrated Brutalism as a utopian and egalitarian style fitting for the modern postwar era. While in the architectural tastes in the West (thankfully) moved away from the stark lines and honest lack of ornamentation, the Eastern bloc continued to use of large ferro-concrete structures. This underscored the economic stagnation of the Brezhnev era and was an apt symbol for a Soviet state that was no longer as competitive or cutting-edge as it once was.

Actually, we can trace almost all Soviet-era architects to the same firm, which explains why there are so many shared characteristics.

First, the rapid industrial growth demanded by Stalin in the years before the war pressed him to pursue the fastest and most efficient building methods available. The reinforced concrete technique that most of these buildings share was developed by Albert Kahn and his brother, Julius, in Detroit in 1895. Essentially replacing wood with rebar, the major advantages to building in this manner were fire prevention and less restrictive interiors. This was due to greater load-bearing capacity in the walls, supports, and floors themselves.

First used in the Packard auto plant, then later in the landmark Ford Rouge facility, the Kahn's firm was at the forefront of American industrial design. In the early 1930's, the Soviet government's American Trade commission, led by Saul G. Bron, contracted the Kahn firm to build the Stalingrad Tractor Factory (1929), the first of it's kind in the USSR. Quickly afterward, the Soviet government signed a second agreement with Kahn, naming their firm the consulting architects of all future industrial construction in the country.

They also established an architectural academy, to be led by another Kahn brother, Moritz, in Moscow. This trained roughly 4,000 Soviet architects and engineers and led to some 500 buildings to be designed for the first Five-Year Plan. These architects would go on to dominate the Russian landscape for years to come, but were in turn governed by a strict standardized building code. The massive urbanization effort dictated form < function, and was relatively effective in addressing the problems of housing and industrial output, if not style.