Because there's this belief that a for-profit company will naturally be more efficient. No idea if that's actually true in practice though. Or if efficiency > the profits the company takes.
I don't know of any large community ran utilities, just small ones. I'm guessing the scale starts being a problem eventually.
TVA and the NY power authority are genuinely massive, government run utilities. Both are also known for pretty low power bills.
They can be efficient, but I ly if the incentives align towards the desired definition of efficiency. If you give a company a natural monopoly and protection from competition. Then it's most efficient way to make money is just to raise rates