Largely because of our fucked up broken electricity market. The wholesale price is always set by the price of the highest marginal producer required to meet demand - which is usually gas. And although gas has dropped back from its ridiculously high peaks of last year, it is still historically very expensive, because of the international situation.
However, at times like this, when renewables are plentiful, the price on the spot market drops like a stone, and can even go negative. This is visible in Octopus’s Agile tariff, where the price changes every 30 minutes based on the spot market price. Go to mysmartenergy.uk and find your region on the Agile drop down at he top of the page, you will see what people on Agile are paying just now, because they are plugged in to the wholesale spot market.
If you are on a standard variable or a fixed tariff, you pay the same price for your electricity all the time, and the price changes every three months. Suppliers - well, Ofgem mainly - set prices based on predictions of market conditions. But the actual cost of producing electricity varies widely - as does demand. And the supply of electricity is variable as well. Gas and nuclear power stations can be turned up or down, to an extent, interconnectors allow us to to trade electricity with other countries, but wind and solar work when they do. The whole job of National Grid ESO is to balance the grid - ensuring that the voltage and frequency are kept within prescribed limits. One of the mechanisms used to do this is differential pricing. Consumers and companies on tariffs with differential prices will adjust when they use energy in order to reduce their overall cost. There is an incentive to use energy when it’s cheaper - that will be at times of high supply and low demand - and to avoid using it when it’s more expensive, times of high demand and/or low supply.
As the amount of renewable generating capacity increases, we will need less and less gas, so prices will come down. Government could accelerate that change, in lots of ways. Easing planning constraints on onshore wind, increasing investment in transmission capacity, supporting large scale energy storage facilities to help damp down the variations in supply which are inherent in wind and solar - to name but a few. And also reform the market so that the automatic link between gas and electricity prices is broken. The effect of all of those would be to preferentially reduce the electricity, helping to accelerate the move towards lower carbon heating and transport.