Because it means there can then be no single monopoly on electricity generation, nor are people forced into consuming power which is being generated in a way that does not align with their core values.

This was the situation in Australia until fairly recently, people wanted alternatives but there wasn't any renewable options, then when the Government changed and pulled rebates for new solar installations, it was even more unthinkable for people to get started.

Also, I'm not arguing for complete and utter decentralization, but there needs to be options.