If we keep our biscuits outside why it becomes soft?

It depend on the temperature and humidy of the air. If the air is very cold and moist then biscuits would become softer. Also the flour in biscuits begins to crystalize after a few days, theoretically making biscuits more brittle. But in many biscuits, the high sugar content masks this process by absorbing water from the atmosphere, ultimately resulting in a soft biscuit.