People invest and save in banks because they think it is safer. Cryptocurrency is a risky investment and you might lose your money if you misplace your private key or recovery phrase or if your wallet/ account gets hacked. The majority of the people are not risk-takers and go with the conventional methods when it comes to investment. Also, people are often reluctant to invest in a new project.
Some large banks are performing well in the stock market and give healthy dividends. They have established themselves over the years and gained the trust of investors. It is not advisable to invest in one asset. Diversify your portfolio with BTC, gold, stocks, and mobile assets (property etc.).