Why did Holmes advice to abandon the case

Holmes believes the case to be a waste of time and money for the people who've retained him. He sees their task or goal as profitless.

When I went into this case with you I bargained, as you will no doubt remember, that I should not present you with half-proved theories, but that I should retain and work out my own ideas until I had satisfied myself that they were correct. For this reason I am not at the present moment telling you all that is in my mind. On the other hand, I said that I would play the game fairly by you, and I do not think it is a fair game to allow you for one unnecessary moment to waste your energies upon a profitless task. Therefore I am here to advise you this morning, and my advice to you is summed up in three words—abandon the case.”