Biggest star attracts people who congregate around them, but not many of them engage deeper. Some do, that's why whenever there's a big breakout hit, everyone else gets some extra viewers. But not a lot of them do.
Happened before with Pekora, happened before her with Korone, happened before her with Fubuki.
You ever hear of a business principle called "The 80-20 Rule," also known as the Pareto Principle? It postulates that, as a general rule, 80% of outcomes are decided by 20% of acting inputs. This force is true in many, many different places in the world, but most importantly for this scenario, it's true of audience retention rates.
80% of the total audience for Vtubers will be taken up by 20% of the market. I bet you that if you were to look at audience numbers, only 20% of Hololive members have streams with regular views in excess of the upper bound of 10,000 concurrents. Gura is one of those streamers, along with Pekora, Marine and Aqua, whose average falls squarely above a 15,000 concurrent viewerbase. Everyone else only comes up to the upper bound of 10,000, or they don't even do that, falling closer to the average of 4-6,000 concurrents.
If you meet a weeb, you'll see that the ones they know fall into that range. Gura is the face that draws the EOP crowds, and then they either stay within that entryway, or they branch out to the next biggest distributors, Pekora, Marine, Korone and Aqua, about in that order, is how I see it going, with Coco getting loyal "Meme Viewers" that show up to her reddit shitposts in excess of 60,000 concurrents, but not actually sticking around for her 6,000 concurrent stream average outside of that.
Don't expect much retention from people who only know Gura. They're here because it's big at the moment, and she's probably the only one still holding onto the EN launch buff by virtue of sheer sub rate. Everyone else in the sub counts is either a rare stream watcher, or more likely just an average person who uses their sub box the same way the average Steam user uses their game backlog; nice to have, but not likely to revisit.