Because a lot of electric grids are too old to handle the increased load.

To translate: When it's hot, air conditioners use more electricity. This is because they use more electricity because they have to work harder to keep a cool temperature.

The reason why electric grids are too old to handle the load is because:

Electric grids were built for smaller populations with the assumption that we'll build more as we make more babies; AND; electric grids weren't built to handle the temperature rise from climate change.