Dirk Gently's Holistic Detective Agency: Hard to Adapt?

Douglas Adams' foray into detective fiction, with his iconic twist of science fiction and extremely British absurdist comedy, was a novel called Dirk Gently's Holistic Detective Agency and its sequel, The Long Dark Tea-Time of the Soul. These books have been adapted into two TV shows, one on BBC4 and one on BBC America. The books and TV shows are all quite different from each other; even the character of Dirk Gently changes a bit between adaptations.
Compare and contrast the book(s) with the TV shows. Why did the shows change so much? Is there something "unadaptable" about Adams' original work?