Why can't you save Zhonya when opponent kill my poro with desthknell outside showdown?

You can flip Zhonya's in response to the Deathknell trigger outside a Showdown, but it won't save the Poro — the death has already happened by then.

Zhonya's is a replacement effect ("if a friendly unit would die, kill this instead") and it only applies at the exact instant the death event occurs. When the opponent's kill resolves, the Poro is dead and in the trash, and its Deathknell trigger is just the result of that death hitting the chain. There's no longer a death event to replace, so flipping Zhonya's now only gets it onto the board.

The real window to save the Poro is earlier: react to the kill spell itself with your hidden Zhonya's before it resolves. If you do, the death is replaced with heal/exhaust/recall, the Poro lives, and Deathknell never triggers (Rule 808.1.d.1). Reaction timing works identically inside and outside Showdowns — any Closed State is fine.