Why could the Bombe not be used to break Tunny messages?

The Bombe was designed specifically as an electromechanical device to decrypt messages encoded by the Enigma machine, which utilized a different encryption process than the Tunny machine.

While the Bombe searched for Enigma wheel settings using crib-based methods, the Tunny machine was a sophisticated 12-wheel cipher device that automated the encryption process entirely, requiring different technical approaches to break. Because the Bombe was of no use against the high-volume, high-grade intelligence traffic produced by Tunny, Allied code breakers at Bletchley Park developed alternative machines to address this challenge. These included the Heath Robinson and, eventually, the Colossus, which was the world’s first large-scale programmable electronic computer, specifically engineered to handle the complexities of Tunny decryption.1