I'm not sure it's really a niche.
They were also scrambling to move all their services away from use of that database in favour of a horizontally scaled system that could grow further.
The query rate that can be handled by a single conventional server are pretty monstrous these days. You'd have to be simultaneously a) maybe top-50 website level load (I'm well aware that there's a lot more than websites out there, but at the same time there really aren't that many organizations working at that scale, much as there are many that think they are) and b) confident that you weren't going to grow much.
Financial systems especially trading platforms need to ensure market fairness they also need to have at the end a single database as you can’t have any conflicts in your orders and the orders need to be executed in the order they came in across the entire system not just a single instance.
This means that even when they do end up with some micro-service-esque architecture for the front end it still talks to a single monolithic database cluster in the end which is used to record and orchestrate everything.