Because state goal is not the advancement of the particular field(s). State goal is to maintain (build) communism within particular county (USSR) and Warsaw Pact block.
Smart people opposed (unrealistic) party goals. so their death was net positive for Communist state.
For details see multiple waves of intelligentsia cleansing during 1920-40.
It would only have been a net positive in terms of relative support for the political apparatus, and even then only in the short term. Retaining political control would still ultimately rely on infrastructure and military force driven by technical support & advancement. Then again it may very well have been such short term thinking that contributed to the ultimate collapse of Soviet Russia.
Smart people opposed (unrealistic) party goals. so their death was net positive for Communist state.
For details see multiple waves of intelligentsia cleansing during 1920-40.