It's never been any different. How did Gordon Brown become PM? It's the way our political system has always worked. The party that forms the government chooses their leader. If there's only 1 contender that person is elected unopposed.
You also seem t have forgotten that she got 60% of the PLP...
They are all elected politicians. As you well know the PM has never been directly elected by the people, other than being voted in as an MP in his/her constituency.