I can break this down for you easily enough. Billy Dee Williams isn't young anymore. He most likely doesn't look like the lean, sexy man he was in his youth. And he has most likely decided that he doesn't want to do live-action roles. That's his choice. He probably doesn't mind doing animated roles because they don't require him to be in front of a camera, and he does have a very pleasant voice. I really enjoyed the in-joke of hearing him play two-face in the Lego Batman Movie. That was a nice nod.
He was in his early forties when Empire was filmed, and is now near 80. Of course he isn't going to look the same, but that could have been used in his favor. The theme of how time changes people, which provided one of the best Force Awakening scenes with the Han and Leia reunion, could have been explored even more thoroughly through a character whose lifestyle aging would have a further impact on. Maybe he could be lamenting his waning powers to attract women, like Jack Nicholson in Terms of Endearment, getting women by trading on his status as a hero, but finds new purpose with a new threat to the galaxy. Or, they could go in the other direction with him traveling further in the direction of the path of responsibility he was on, and at some point in the 30 year gap found a nice woman, settled down, and became a father (or grandfather), and now approaches this new threat from a different life situation. Wherever they decide to take the character, his finding out about Han's death could provide interesting fireworks, and if he's really that upset about his looks, they could always do like they did with Peter Cushing except instead of recreating young Lando, they could create an ideally aged version of himself.