I cannot give you all the particulars but it has to do with the monopoly status they have and how they are allowed to structure their earnings.

The story is something like this. Because they are allowed to have monopoly status, their earnings are structured in how they are allowed to set rates (and in turn profit). The way it is structured is they are allowed to cover their cost (including debt service) and then turn profit on their invested capital. So to maximize profit they leverage out and invest in bigger and and more expensive power generation plants. More capital invested = bigger profits = higher stock price / dividends