There are a number of reasons that could explain why we’re only just beginning to see a new generation of core banking systems emerge:

Banks generally move slow. They have a historically cautious nature, which is understandable given the criticality of the data they are managing. There is typically a lot of governance around IT change in order to mitigate risk, with the unintentional side effect of slowing things down

An effective development ecosystem and culture can take years to organically grow, which means banks find themselves with a dilemma, as key skills like Cobol are disappearing from the marketplace, and transplanting a new in house team focused on a more modern development ecosystem can be a long term endeavour

Up till recently the cloud generally wasn’t trusted by regulators, inhibiting banks from leveraging cloud infrastructure to lower the entry barrier to scalable infrastructure