Yeah, it is decided through combat, but since the founding of Wakanda, T'Challa's bloodline have been the constant victors, and their line has been imbued with the ability--either a genetic deviant or a gift from the Panther god, to be able to metabolize and handle the herb. Killmonger, though in great condition and with enhanced stats, almost died when he ingested it.
T'Challa's family tree has the best combatants--so bloodline and combat ability have kind of blurred.
