Electronics require power, And when the power runs out or the electronics malfunction the cash is still fine. Electronics have been shown to malfunction quite frequently. Card readers have been shown to constantly malfunction, For example. Now, What if you don't have access to any electronics? You won't be able to access your money. You'll need constant access that physical money provides.
Mainly as cash is by its very nature - physical. You can use cash digitally but it will always come a cropper in some circumstances. Such as forgot your mobile or card. Or the service only takes physical cash. There are costs and infrastructure requirements to digital cash that are not there for physical cash.