So I worked on one of the major banks' share trading platforms for a couple of years. They are written in such a way that they only cater/assume one exchange (well this one was and I suspect most of the older ones are the same) because we've only ever really had the JSE in the digital age.
Early on, before it even became public knowledge, they got wind of the fact that a second exchange is coming and from the technical side preparations (planning, high-level designs etc) started so that changes could be made to cater for multiple exchanges.
But what happened? Politics. The thing with corporates is that there is this ladder. The guys climbing the ladder basically shovel the **** thrown on them back down to the guy below them (colourful analogy, I know). They care more about the short-term performance of the systems in their portfolio so that they can "look good" in front of business, get promoted and make the existing issues the next guy's problem. Rinse and repeat.
In this specific case we had an extreme version of this. The guy deprioritised the work so many times and for so long that it never happened. I left, two more guys left, the dev lead/architect left (basically the core dev element left) and the guy was "promoted" sideways. So a major share trading platform is sitting with a new dev team with little domain knowledge on a very old codebase and their first priority will be to keep the system (with a huge backlog of issues) running.
And that's why ZarX is not supported. I'm not saying this is true for all of them as the above was a bit extreme, but if I had to take a gamble I'd reckon that EasyEquities will probably be one of the first to support ZarX because their system is new. ABSA might beat them to it because I think their system is an off-the-shelve product which they customised.