Answer:
there were diminishing returns to capital.
Step-by-step explanation:
The single most important factor that determines the long run growth rate of an economy is productivity. When productivity increases, you are able to obtain a larger output using the same amount of resources. Generally in a national level, production increases result from increasing human capital (more education, more training) or technological innovation like automation that increase total output while keeping decreasing or keeping inputs in the same level.
In macroeconomics, when only one factor of production is increased, e.g. capital in old USSR, while the other factors (land, labor, technology) remain constant, the output per unit of the increasing factor will tend to diminish. In other words, you will need more capital every time to increase output in the sustained manner. E.g. in order to increase productivity by 1%, you need 2% increase in capital, but to increase productivity by 2%, you need a 5% increase in capital.