Note that the Marxists-Leninists themselves preferred Lamarckian to Darwinian principles.
The idea was that Lamarckian inheritance showed a way to create the New Soviet Man. Trofim Lysenko provided a biological explanation for the creation of the New Soviet Man- if they simply worked hard enough at the creation of the New Soviet State, then the New Soviet Man to live in the New Soviet State would come into being. All that was necessary was personal will-power to live your life in the proper Marxist-Leninist way[1]. You see, the unguided nature of Darwinian principles bothered the Communist theoriticians; they much preferred the directed, guided Lamarckian inheritance to the random mutation-natural selection that Darwin offered. They could control Lamarckian principles, they couldn't RMNS.
Why did the theoreticians and not the scientists make the decision? In 1929 Stalin gave a speech announcing that in the future the apparatchiks would control science- it would not be left to the scientists but vetted for its political and philosophical agreement with Marxist-Leninist theory. By 1935 Lysenko had gained control of the Soviet agricultural and biological apparatuses. During the life of Stalin he crushed all opposition to his politically proper but scientifically invalid ideas. At least one of his scientific opponents was killed by the NKVD during the Great Purges of the late 1930's and early 1940's. In 1948 he gave a famous speech denouncing Mendel's theories as reactionary. Lysenko maintained his grip on Soviet biology until ~1965, when the tide finally turned and the scientific reputation of Lysenko was destroyed. But Soviet biology and genetics never recovered from the disastrous 30 year interlude in the land of politically-controlled science.
Just a point to make on this topic,
Chris
[1]: This was one of the many justifications that the Soviet State offered when they shipped undesireables (the Parasites, in the jargon of the day) to Siberia- it was a quarentine to keep them from reproducing their degenerate ways in the next generation. Obviously many other justifications existed, and the biological one was not the reason for the Purges, but it is a reminder of the way that people twist science to support their ideological goals.