because banks always hate cryptocurrencies and think that electronic money is an unclear kind of money roughly "black money" for money laundering criminals so banks and laws in many places are still elusive. Concerned about the long-term viability of BTC, because BTC has no stable value and at any moment it will burst like a bubble.
Aside from that, their clients will find out that dealing with crypto will incur them very cheap fees especially cross-border payments. Fiat transfers are still expensive as compared to crypto. And also, since the government can't control all the transactions in crypto, for them, there are still blind spots and they don't know how to tackle it yet. But I guess, slowly, the concept of blockchain technology will be applied to banking industry.