It's a combination: some users hv dropped out of card collecting/dailies; some users aren't as active (on KS as a whole); trade threads are more prevalent nowadays & I think some are too busy/it's too low ROI to make singular threads talking about trades anymore(I found tat too much effort for not much engagement in return after a while making singular thread). Also I stalk profiles/trade threads so I don't make threads talking about cards much.
I agree with this and Soogi, I stalk people's trading centers (mainly Nina and Hakku) to know what they're looking for to see if I can get anything they want for something they have and are trading. My go-to is never to look for individual threads, as most of the time they are outdated anyway. I go directly to people's profiles looking to see if I have something they want and if they have something I want. Most of the trading offers are done via user's profile pages or designated trading threads if the user has one that is easily accessible.
I wouldn't say the game is dead, especially because there are dedicated users like the above quoted user who actively seek out trades and even trades that involve three users trading in a triangular fashion. There are also new users just getting in the groove of things, so posting a thread to find users who want their idol X is something they may figure is like talking to a wall. It took me a while to realize you could even post a thread in the Card's Care Center.