In ESB Lando was a retired scoundrel administrating a mining facility and living a tranquil life. Maybe after being a general in times of war he just decided to go back to what he endeared most. After all, he betrayed Han cause he wanted to keep the empire away his city for the sake of his people. Perhaps he even returned to Bespin. He doesn't need to follow the rebels or Han and Leia wherever they go. They're friends but each one to their own. But don't worry, I'm sure we'll hear about Lando in one way or another, in one episode or another.