This is a confusing concept but if you think about it, it is not at all complicated. First, we must realize that electrons are not in some narrow orbit but occupy a general space like a cloud. Each space has a certain amount of energy for an electron to remain at that level. In, addition electron energy is measured in EV (electron volts) and it is a negative quantity.
As you can see from the reference link, the closer an electron is to the nucleus the greater it's negative energy. It must gain energy to jump to a more distant level. But like adding positive and negative numbers, as a negative gains some positive quantity, it gets closer to a zero (neutral) potential. That is why most electron transfers occur at the outer levels where the energy required for movement is less. For an electron to move from an inner energy level (potential if you will) to an outer one requires a great deal of energy, although once the first level has been overcome the next levels are closer in energy and require less input to make a jump. At the outer levels of any atom or molecule the potentials are much more even or closer together, so change happens rather frequently and is much easier at that point.
I guess the easy answer would have been one related to entropy, that all reactions in the universe tend to slow down and go to lower (not negative but zero) values of potential. But I thought you deserved a bit more.
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