Why was Indian civil service call the back bone of british rule in India?

Because given the sheer size of India and the corresponding necessary size of a civil service, it would have been impossible for the British to rule India through a civil service that they would then have had to import from Great Britain.

As it was, the Indian civil service adapted well to British laws and regulations and to Western-style administration. This made it possible for the British to have a efficient administration of India with a minimum of Staff imported from Britain.