The major cloud providers all aim for 99.999% uptime. Keep in mind that 99.99% uptime means ~4 minutes of downtime a month. I think there are other reasons that banks may not want or have the ability to run their core services on the cloud.
It’s atypical motivation but one of the few verticals I know of where the cloud is largely out of the equation.
I think Azure is cheaper for certain workloads as well, and at one point had DCs in places where AWS didn't. But it's mostly the "we already buy X Microsoft product, and they cost about the same, so..."