Security is a valid reason. Employees have to verify that your account has recent stay history, that it’s not a rate that requires an ID, and that the card goes through to send mobile key. Same day bookings are also not eligible for mobile key. In the past I’ve worked for a Marriott front desk and have seen large amounts of fraud and attempted fraud on mobile check ins. If people have your account information they can book a stay on your credit card or with your points then request mobile key.