It's pretty obvious why Lendl left Murray and why he won't come back. He's afraid of facing his nemesis Boris Becker. All was fine when Marian Vaida was coaching Djokovic, but now that the German has joined Novack's team...
Becker, the only guy who could out-bully the Tjech bully, still gives him nightmares. Lendl will only coach Murray again when Becker has left Djokovic.