Historically, Sunday has been a day of rest. A holiday in most of the Christian countries around the world. Banking was done during the 6-day work week before the 5 day work week was adopted in many countries, but Sunday was always off.
As the world got more and more electronic and connected, you have different banking systems, networks and a whole lot of legacy code.
Imagine you are asked to build and code a system in say the 1960s and keep building on top of it, you can imagine what a spaghetti and clunky system you will even if you keep changing your systems every couple of years, which resulted in disparate systems not very willing to connect and talk, unless 3rd party standards came out and still it was a pain to connect them.
As time progressed, it made sense to follow the rule of Sunday being off, and banks earned a handsome income on the money held in deposits (they could lend this out for overnight lending, etc.)
Newer banks and new economies, had an advantage of not being burdened by antiquated systems. So it was easier for them to connect and execute. Today, world over, almost all the banks, are being forced to connect (by regulation/central bank initiatives) or by regional collaboration to network switches to enable a more 24/7/365 operational mechanism.
These systems rely more on automation and believe in minimal support staff, whilst giving preference to transaction types that make sense to be executed 24–7. For example not many users want to open a letter of credit on a weekend, so hence, no L/Cs are offered on the weekends. However, person-to-person payments are more likely in demand and those transaction types are handled.
Most countries have a 24/7/365 payment system that for example connects all the ATMs or POS machines together. It is the transaction type that is important. Not many banks see the need to implement a real-time system (including Sunday) for clearing of cheques (as they deem it to be a dying proposition).
Implementing 24/7/365 switches is not about technology. It is about operational readiness, regulations, demand/urgency and are relatively expensive to implement (here is a slightly incorrect map of all the countries in the world that have a 24/7/365 payment system in the world.