To be honest, regarding both this election and US primaries, the candidates are the party's, I don't really see any reason why anyone other than party members themselves should get to say anything about who leads them. Parties submit their candidates, anyone can register as a candidate in the general election, and everyone can vote for one of those candidates. Explain to me why a non-party member should have a say in who their leader is (let alone just one group like union members).