It's because power needs are reported as watts, and impactful devices are typically utilized for several hours. It's much easier to do your monthly utilization napkin math in multiples of kilowatt-hours than of 3.6 megajoules.

I'm personally not a fan of the kWh, because it keeps being mistaken for kW, but I recognize that lack of need for a conversion factor help a lot of people make sense of energy.