1. Banks can legally spend around 90% of the money you give them on their firther investments. They just rely on the fact that usually people keep their money in the bank and don't bother to suddenly withdraw all of it. When they do, it's a catastrophy, bankrupcy and losses of many people. Another thing is banks know your identity, so they can keep track of what you spend your money on. Moreover, they keep charging you for all sorts of things like balance updates, cash withdrawals, account support etc.
2. You store bitcoin in a wallet that belongs to you, so nobody will use your money ever for their own needs, and in the event of instability when people decide to use their money in panic, you have 100% of what you had in btc. The wallet is not connected to your name, so while people can track the transactions, they don't know who you are. This gives you some privacy, at least. Moreover, you only have to pay for tranfers of money from one wallet to another which is usually cheaper if you consider all those payments associated with the bank account.
3. If the first two points didn't provide enough evidence for you to care, then probably rational arguments just don't convince you. However, I think that these people don't have to be convinced! It's the big companies, chains of supermarkets, the most famous online shops etc. that have to be convinced. And I am pretty sure they care about cutting the expenses and enjoying bigger profits. The small ones will follow once they see that the customers of the big ones are already used to cryptos.
So why has it not happened yet? Well, because the market has to recover and stop being that volatile to be taken seriously. Apart from that, bitcoin is probably not even suited for mass adoption, because it cannot handle enough transactions per second. Other cryptos can, though, and bitcoin could still be used in some areas.