I track a bunch of K-pop artists on Spotify for my milestones thread: [ONGOING] Spotify Stream Milestones for Korean acts
One thing I'm noticing is that outside of the top groups, most do not have "good" streams on b-sides.
Now for K-pop, "good" is at least 10 million. Some acts with over 1 billion streams are barely scratching that marker with 1 or 2 b-sides per album, and some acts take years to do that. Now, it needs to be said that most acts that debuted in 2012-2016 won't get the same kind of hype for the older albums. But even for acts that debuted in the 4th gen, that's not happening.
Why is that? One of the main reasons I can think of is that a lot of casual listeners will check out the title track of a group and maybe a promoted b-side if it gets a video and/or live performance.