Multiple reasons. Firstly, Britain (a world-wide power) was setting humongous taxes on imported and exported goods. (As in the 'Tea Party' the Americans rebelled by throwing the tea into the sea). The colonies were irritated by this whole control, and wanted more power and say in what taxes were put on them.
British disregard of religious beliefs and practices.
A rumour was spread that the riffles for cartridges were made from cow and pig fat. Cows are very sacred in Hindu customs, and are not eaten. Hindus reveres and worship cows. It is considered a sin to kill a cow and eat its meat.
