Both of these are important to emphasize in a textbook chapter, as both of them are fair representations of the role that communist policies had in the Soviet Union.
If we only reported on the achievements in space of the Soviet Union, then we would minimize the damage that certain policies caused. On the other hand, if we only reported on Stalin's Great Terror, then we would be painting a biased and inaccurate picture of the regime. Therefore, it is important to mention both of these events, as well as the many others that defined the Soviet Union during this time period.