I think one key thing about the European leagues is that they limit the number of Americans on each team. So since Lucca is German, he has a bigger 'market' available to him there (in terms of roster spots available) than any American players do. If he's contributing on the junior national team and has a shot at the real national team, it stands to reason that he could be a good player in their pro league. As others have mentioned, he may have come to the conclusion after his year of DI bball that he doesn't have a great shot at the NBA. If this is true, and especially if his knee may be the limiting factor in his pro career, it would make a lot of sense for him to bolt back to Europe. That said, I really thought it would have made sense for Craig to go pro this year too.
Regardless of the factors behind Lucca's decision, I REALLY hope he stays. I think he could play a very special role on an ISU team with all the right pieces in place.