The Unions hate democracy. This whole story is about the Unions being told they must behave in a democratic fashion if they want to strike and they are up in arms. "How dare the nasty Tories take away our undemocratic right to call a strike when ever we feel like it".
When voting for a strike there are only two option to vote for, yes or no. When voting for a MP there can be many candidates standing and the votes are shared out among them. If only two candidates were allowed to stand then you would expect a higher majority.