When the British came to the subcontinent, they labelled everyone who wasn't Muslim or Christian as a Hindu. Sikhs, Jains, Buddhists, Vaishnavis and a multitude of others were labelled Hindu. The main influential group among these Hindus were the Sanatan.
As the British were exiting, the Sanatan found it to their benefit to keep this British label because it made the domination and subjugation of other faiths easier. However, these other groups resisted the label that was cast upon them. The Sikhs were very vocal about it. One by one, the Sikhs, Jains, and Buddhists managed to separate themselves from the Hindu label. But the other groups, like the Vaishnavi and Shaivi, were unsuccessful.
In the south of India, you will find many groups still claiming "we aren't Hindu" but have been unable to separate themselves (especially as the dominating Sanatan refuse to accept their separatism).
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en.wikipedia.org
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(Published in 1899)