The major obstacle is the way the Indian National Congress developed. Once it became solely about the establishment of a pan-Indian state, it will be hard to justify multiple states. certainly the critical time period is between 1885 with the establishment of Congress and 1920 when Gandhi became dominant.
Instead of Congress becoming the sole voice of Indian national aspirations, you likely need a variety of regional organizations. Maybe British policy changed after the Sepoy Rebellion to divide India. So instead of one raj, there are several. Or the British don't allow Congress to exist, but tolerate smaller, regional organizations. Or around the time of WWI, Britain puts together a credible dominion status plan for India, but on a more local level with certain regions of India getting the chance for self-governance sooner (which would enourage people in those areas to not support a greater India). Certainly, Muslim regions of India might prefer that to a greater India.