When Kethis was first revealed, it was the most expensive single card in Core Set 2020 at $25. As you see, the price got slashed significantly, and understandably so.
Just like Kaalia, this is exclusively an EDH Commander card that won't see much play in any other format, hence the sharp price reduction that happened right after the massive initial hype.
His second ability looks mighty good in the right kind of Commander deck, but if it works as intended needs to be tested. That is why the current pricing on Kethis won't change much in the near future.