The order needed to use Sherlock Holmes in order to solve many cases because he was without a doubt the best and most skilled detective in the world. Part of his ability came from the fact that he was actually only half human. He was the product of a mix between a very human like life form and a woman from Earth. The alien species is a hunter species that is able to track down anything by analyzing the paths and the areas that can lead them to find their prey.
Sherlock Holmes solved thousands of cases for the order and he was alive for much longer than people actually thought. He had to have a staged death at a certain point but he lived to be 150 years old. There has never been a detective quite like him and even other people who have mixed blood with that alien species have not been so gifted.