I think three reasons may be focused in this case. 1. acceptability 2. credibility and 3. availability... these three factors make them whether superior or inferior.
but cash is only accepted in a selected area. for example a cash can only be use within a state, while usng t in another country you need to change your cash into into local currency where you want to use it. while bitcoin is a universal currency and it can be use anywhere in the world without changing it into local currency.
Also Bitcoin will not for micro transactions, to buy something thats worth a few cents, no. Bitcoin is specialised, and can be better than cash in terms of security, and you can expect its price to continue to rise