Not talking digitals, the group direction seems to be more towards fandom rather than public. Their songs are quite loud and fast paced, a lot more than what the public seems to be into. They were def aiming for more fandom, hence the pumping out of content for fans.
They had consistent promotion and comebacks, they had a budget and their own sounds and all.
No scandals that I know of, stayed the same concept, etc. Usually those can be a pit fall, changing concept is risky, waiting too long for a comeback and fans leave, scandals and members leaving causes losses, but none of this happened. Even by the first comeback they had lost sales. Usually for nugus they just stay at the same meager amount, not bleed.
The fact they started going downhill by the first comeback, without any of the hallmarks that you can usually correlate to losing some fandom is wild.