They had ruled all of the Indian subcontinent for nearly a century despite its diverse demographics so what pushed them to simply carve up what had been one united political entity. Did they do it just to weaken the post-colonial nations or did Indians themselves want it?
The question has recently been asked. Maybe check out this thread: https://www.reddit.com/r/AskHistorians/comments/z416qs/why_did_the_british_partition_india_instead_of/