The way I have always tried to explain it is that the dark side is like a drug. It’s hard for us to understand, but once Luke really taps into that raw power, an ability where he can bend the universe to his own will at his very fingertips, he’ll be hooked. Once he gets a taste of it, he’ll just want more and the Emperor would be like his drug dealer. The Emperor so firmly believes in the temptation of that power that Luke turning is without question. So when he doesn’t, it is a complete surprise. And who knows, Luke might have turned if he had not noticed the parallels between Vader’s robotic hand and his own, seeing in an instant that he was becoming his father.