Lorang was fairly clear about the reasons for not selecting Buchmann in his
preview on Cyclingmagazine. The team aims for a GC podium and stage wins, and Buchmann is unlikely to achieve either of it. If a climber had dropped out, Buchmann would have been 1st choice to come in instead, but to replace Welsford, Gamper was the more logical choice for them, as he can help van Poppel as well as Martinez.
On Wielerflits, Gasparotto also said that no one ever promised Buchmann a leadership role. According to that interview, it was between Koch and him for the spot in the team., and they decided to go for Koch, as he potentially can win a reduced sprint in a breakaway stage.