If I have to point at one thing, it's Selen's termination
and its aftermath, specifically the infamous black screen video. But like exploding tnt, it's only the spark. The fuse has long been lit. I'd argue the biggest factor of Niji's current struggle is their incompetence of managing foreign branches, going as far back as Niji India.
Each foreign branches seemed to be left on their own devices, seemingly without supervision and that much support from HQ. Niji ID was very successful back in the day, with livers like Hana being one of the very few big English-speaking vtuber. They came out on top of the competition on the local market, but the HQ never really cashed in on the success while holo ID starting to catch up. By jove, Hana only received her 3D model last year, and that was after multiple delays.
As for Niji EN, if the rrats are true about its management (or its lack thereof), then who's really surprised when it all crashed down? Having a circle of friends doing something together is nice and all (like early vshojo), but it doesn't work that well in a corporate environment.
TL;DR Niji doesn't have the commitment to maintain their foreign branches