Re: It's not "why", it's "how"
"Here in the UK we had people claiming Gordon Brown was never elected as Prime Minister and again, that Theresa May was not elected as Prime Minister."
To be fair, neither of them were. What people don't understand is that they don't vote for a PM they vote for a party, but the political system has become so personality driven it's hard to see the difference. Once the leader of the elected party steps aside the party can choose a successor, its just that if it happens to be in government we end up with a new PM.