Because the money you have in the bank is not your own, it's the banks. They make insane profits for that convenience while offering a lousy service. Oh, yeah, and when the run on the banks happens they will all go bankrupt. Want to know why? Because of the fractional reserve banking system which gives banks the power to hand out 10x more money in form of loans than they have "in the vault". So, when a big amount of people ask for their money back the banks will just declare bankrupcy. Leaing you with 0 fiat currency in your bank account. And you won't be able to do anything about it.

Unlike in the scenario of banks dealing with your money and making profits, if you have it in cryptocurrency at least you own it. And you can say that its really your own. And no one can take that away from you.  

While I wouldn't use bitcoin for everyday transactions as it has proven to be very slow and costly when the load on the network is high, there are many other coins that can be used as a mean of transaction and will be able to handle millions of transactions a day without breakng a sweat. With 0 fees and 3 second transaction times.  Wonder about whih magic coin I'm talking about? STEEM :)