I believe the answer is: Biases
The person who made such comments usually had a a really strong preference in one economic system, to the point where that person refuse to acknoweldge the weakness in their preferred view and the strength in the opposing view. This bias could easily be held by all sides.
This is not true! There are many other such factors.
One of them is similar to food: the availability of water.
Other is also natural disasters: tsunamis, earthquakes... they also limit population from time to time. Similarly diseases such as Aids.
