I can't say for sure, but I think it would be something like reputation, brand power, and consistency. Their reputation was never the best, but with Burning Sun, BI's following scandal, and both of the Yang brothers stepping down, I'd say it's pretty much obliterated now.
And that's reflected in the drastic decrease in stocks.
As for their acts, Big Bang is over. iKon will continue as six, but I'd say that after losing their leader and composer, they're quite shaky. I don't know how well Lee Hi did on her recent comeback, but the rest of their artists, save Blackpink and Winner are too underpromoted and unknown. Plus, if what MonCherry has said about their actors and artists currently trying to get out of their contracts, then we don't even know exactly how many people they'll still have under their agency after all of this is over.
Going a bit off track, I wouldn't call BigHit a big 3 company as they're still relatively new and haven't proven their consistency yet. YG isn't really in the same boat, but I don't think they've had consistency for a while now.