Mango Airlines voucher versus full refund

We booked Mango flights to attend a graduation ceremony. Due to the current coronavirus situation the university graduation ceremony has been cancelled with no future date. Man go airline will not refund the money paid but only offer vouchers for future flights. All other service providers, car rentals and the bed and breakfast, have offered a full monetary refund and not vouchers. This cancellation is a force majeure event and affects everyone in SA, and it is certainly not in our hands. Why will Mango Airline not offer a full refund?