Because someone owning an oil fired power plant can buy all these negative priced oil futures, take actual delivery, and burn the oil to produce electricity and get paid for that.
Normally, burning oil to make electricity is uneconomic, since gas, coal, and even renewables are cheaper. Oil fired plants were sitting mostly mothballed for the last decade in most of the world, for use only in emergencies.
To mirror fauigerzigerk's point: The power demand is already being met by the existing system. How could you profit by burning the oil immediately for power generation?
It's true that low oil prices will encourage oil fired plants to open back up,but I'm not sure that a very short term drop is going to have much effect since spinning up and down a power plant isn't exactly a short term decision.
Normally, burning oil to make electricity is uneconomic, since gas, coal, and even renewables are cheaper. Oil fired plants were sitting mostly mothballed for the last decade in most of the world, for use only in emergencies.