The “ecological footprint” left by a citizen of a developed nation is about four times larger than that left by a citizen of a developing nation. why is this the case?

You can compare this to the analogy of climbing on top of a mountain. When you make a few delays when you are still near the base of the mountain, it would not significantly make an impact. But if you are already near the peak, it's do or die. Every act you do is significant because you are already towards the end of your goal.

So, since a developed nation has a very high economic power, it also takes effort to maintain it. Thus, every act must be consistent.