The reason is every one knows the value of cash and they can hold it. Every one use it to everything they want. Zero fees, no need internet to use it, all business establishment accept the payment of cash even on board on the car, bus, ship or any means of transportation. In bitcoin you didn't do that.
Agree and cash has no requirements to used unlike bitcoin if you do not have gadgets and internet you can not able to have and used bitcoin, that was the main disadvantage of bitcoin but in terms of investment much bigger income go to bitcoin.