Here's why:
* The Power Generator is destroyed in the game's story. It's the main reason why the Kanto region is in such a state of disrepair.
* The destruction of the Power Generator is a plot point. It sets up the need for the player to fix it, which involves collecting the eight Badges and facing the Elite Four.
* There is no way to repair it. The Power Generator is a key part of the story's plot, and the game doesn't provide a mechanism for repairing it.
However, you can use a Power Plant that functions similarly to the Power Generator in Kanto. This is located in Ecruteak City, and it provides the power necessary for the region to function.