I know as little as the next person why Putin hasn't shown up, but I suspect he never intended to.
If you think about it, he never actually said he would go to Turkey today. He simply commented upon a meeting being arranged there.
He was hoping either to put Zelinsky in the wrong, if he too, does not go to Instanbul, but only to Ankara today, or to keep him hanging around waiting. In other words he wanted to demonstrate his contempt of Zelinsky, whom he and Russia generally have been calling incompetent, undemocratically appointed (nice one comming from Russia!) a Nazi and the person responsible for starting the war.
Second guess, not only mine, but journalists and military experts in Denmark, and I imagine everywhere else: Putin does not want this war to end until and unless he can persuade Russia and preferably the rest of the world too that he has won it.
He is not interested in a cease-fire, or in peace. He wants the entire Ukraine back where he thinks it belongs as part of Russia, control of the Black Sea and probably a treaty with Turkey stating that Russian shipping can at all times sail through Bosphorous, but any other nations that want into the Black Sea do so only on Russia's say so.
There are always rumours that a dictator is dying - I vividly remember the year leading up to Franco's death, and how often and for how long Fidel Castro was said to be dying!
My parents could remember rumours of Hitler's and Stalin's death ages before either of those two died, and I imagine my great-great-great-grandparents heard rumours of Napoleon's while he was still First Consul.
