First of all bitcoin isn't a currency at all , I agree it is accepted in many places where we can buy goods or services with bitcoins but of course with limitations. Bitcoin is a investment platform where one can invest and 90% of the time he will get a profit after few months depending on the price he purchased at and also if you are saying that why bitcoin when cash is there then why do people invest in gold when cash is there. Because gold has a potential to increase. Same for bitcoin it is more like a investment platform and it has a potential to increase in the future unlike cash which has the same value for years.
Actually cash loses value over the years and doesn't have the same value for years.
It becomes less useful to use Nation's cash because you need to haul more of it to do each thing.
in the 1950s Used to cost $0.10 dollars for 2 pizzas and a soda in New York City... today it costs $5.00 for just the slice of pizza.
In Bitcoin it once costs 10,000 bitcoins for a pizza, and today it costs 0.01 Bitcoins for the pizza... you could see where things are going.