I am spending some time in the Himalayas and wondering why Simla ended up as the center of British power among the hill stations? Darjeeling was much closer geographically to the capital Calcutta, and has a similarly pleasant climate. I finished Dane Kennedy's excellent book The Magic Mountains, which doesn't seem to answer that specific question.
Simla was declared the Summer Capital of British India by Sir John Lawrence, Viceroy of India in 1864.
Sir John’s career in India had always been based closer to Simla than to Calcutta.
He came to India in 1829, and was based in Delhi as a magistrate and tax collector (for the East India Company). He later served in the Punjab, organizing supplies for the British Army in the First and Second Sikh Wars, and serving on the Punjab Board of Administration and as Chief Commissioner of the Punjab. In the Indian Mutiny, Sir John led the forces which recaptured Delhi from the sepoys.
After the Mutiny, Sir John returned to Britain, but was sent back to India as Viceroy (India now being governed by the British government not the East India Company) in 1863.
https://en.wikipedia.org/wiki/John_Lawrence,_1st_Baron_Lawrence
It appears that Sir John felt that the center of British interest in India had shifted West, to the Punjab and the Northwest Frontier. Calcutta was too far to the East.
To him it was not Simla’s beauty that mattered but its strategic geographical location. He said of the choice of Simla as the location for the summer capital, "You have one foot in Punjab, another in the North West Frontier, yet near enough to influence Oudh".
http://www.tribuneindia.com/2014/20140706/spectrum/main2.htm