It's not THAT easy. But the process is their tradition. Remember, the ruler of Wakanda is also given the powers of the Black Panther and is expected to protect the country from outsiders etc - so combat skills are definitely a requirement.
However, when a King dies, the King's first born takes over - much like in any monarchy. The big difference here to most monarchies is that they invite open challenges from the tribes that make up Wakanda's people. Each tribe has to nominate a challenger if they wish to challenge for the throne. None of the tribes wishes to challenge T'Challa's claim except for M'Baku of the Jabari tribe.
Killmonger has the ability to challenge for the throne because he is "of royal blood" - he's the son of T'Challa's uncle, who was King T'Chaka's brother. If he wasn't of royal blood, then his challenge couldn't be accepted and would be deemed invalid.
Conor McGregor couldn't take a shot at it as he's not Wakandan, so he couldn't be nominated by a tribe, and unless he could prove some royal connection to T'Challa, he's an invalid challenge as well.
