Are you a new host - if so, you may be one of the unlucky test group who are not allowed two switch off instant booking.
For AirBnB it is clearly a revenue generator as it encourages impulse buys and has no reliance on the host repsonding to a request.
Of course it means the host are left wide open to scams, people who they would not normally approve being given access, and no way of engaging in a community with the people coming into their homes...pretty shocking stuff..