The gates at T5C (and T5B) are not equipped for regular bus use; the stairs in each node are only used by passengers when there is an airbridge malfunction or emergency (they are classed as back of house), and accessibility access to the ramp is poor (the lifts require staff operation). There's also no holding area within the node or at ramp level to board the buses.

There's also the possibility that all gates were in use with other arrivals or departures.