I’m unaware of any Core Banking Systems (CBS) that runs on a cloud provider with the exception being Finastra (Azure). Other parts of retail banking stacks? Sure. Not their cores.
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..."