Governments are extremely bad at calculating how much something "should" cost. Profits are revenue minus expenses. "Expenses" are revenue to someone else. Whenever you try to cap "profit" of some entity that isn't actually subject to competitive pressure, the next thing that happens is that the people running it start colluding or taking kickbacks from any of the people whose revenue constitutes their expenses.

Because then the utility company's expenses go up, which they would normally not want because it reduces their profit, except that then they can say that their profit isn't over the threshold and use that to increase revenue (i.e. raise prices) and still make the same profit but now also get the kickbacks under the table or help out their cronies by overpaying them.

It also in general deprives them of the incentive to improve efficiency, because reducing waste lowers expenses, which in a normal system you have the incentive to do because it means you make more money, but not if profit is a fixed amount defined by the law regardless of what you do.

Which is to say, it's much better to find a way to narrow the utility monopoly as much as possible and subject them to competitive pressure as much as possible, than to try to pretend that you can limit the profit of an entire monopoly supply chain by declaring that a single entity in it can only have a specific amount.