There were a variety of factors that led to the British withdrawal.

a) Depletion of military forces

The UK, like most other powers, had their military strength significantly depleted by the war. They needed to maintain troops to occupy other territories, and the native Indian troops were growing restless.

b) Rise of Indian nationalism

Indian nationalism took two forms: the non-violent form supported by Ghandi, and more violent kind favored by Bose, who later joined with the Japanese and formed the Indian National Army, a force of former Indian troops that the Japanese assembled to fight the British. After the end of the war, these Indian troops were tried by the British, but ensuing mutinies by Indian forces in the army and navy, as well as popular support for the INA troops, removed any hope that the British had of maintaining the Raj. In fact, Clement Atlee, the PM, specifically designated the INA's activities as having jeopardized the British ability to hold India. Many former members of the INA ended up taking various roles in the post-independence Indian government, especially those more deeply involved with the movement alongside Bose.