Because getting cheap power for 25 hours doesn't fully offset the price of gas for the other 30 days in december. And when you say "uk energy companies" do you mean the wholesale suppliers who sell you the electricity (many of whom have gone bust), the generators who make the electricity, or the global multinationals who sell them the gas with which to make the electricity.

Because the only one making bank are the multinational oil companies that get to pump the gas out of the ground..