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Sankisei gets the title "Generation of Miracles" because they are composed of veteran streamers who cover a variety of niches and are willing to lean into their characters' roles, with a remarkable sense of unity (prior to 3rd gen there was no such thing as 'gen unity' in Hololive), smart enough to coordinate their schedules and not overlap. During their prime, the 1800 - 2300 JST slot was dominated back-to-back by 3rd gen, which allowed them to cycle the viewers into another Holo stream and kept the viewers' interests in Hololive only.
Other factors include the death of .LIVE led to a power vacuum for viewers who like to watch cute girls doing cute things, and 3rd gen was quick on the uptake and played up the unicorn fanbase. The JP called them the "Cabaret Gen" for a reason.
They are also very balance in terms of streaming style due to their experience. Marine was very popular on NND. Pekora wasn't as big, but she was put through the fire against misogynistic trolls and understood how to deal with hostile audiences. Flare has been active around the underground music/creative scene. Rushia has been streaming for years as an indie and a few minor corpo Vtuber persona. And everyone knows who Noel is. She was probably the most famous hire Cover ever has.
Marine and Pekora are some of the best vtubers in the industry. Pekora floundered a bit at the start of her carrier, but eventually got her breakout moments with PekoMiko. She also has an almost pathological desire to stay on top and maintain her position as the top Vtuber. If you notice, she almost never plays any long-series game that might jeopardize her number (with the exception of Crayon Shin-chan and Xenoblade Chronicles). Marine knows her audience very well and will never act in a way that would turn them against her. Rushia has no remorse milking her paypigs and plays into the GFE role extremely well. Noel is actually pretty funny if you watch her streams, and her boobs will always attract viewers. Flare might seem boring, but as a streamer she has pretty decent stats (can sing, her playthroughs are decent, good connections for collab).
tl;dr - experienced streamers, excellent planning and coordination, desire to improve themselves, and a few lucky breaks like .Live collapsing
