What we need in the US is tough enforcement of the existing labor laws. The Wage and Hour Act, classification of employees vs contractors, and the right to organize are all in US law, but the penalties are weak. Did you know that you have the legal right in the US to organize a union whether your employer likes it or not?
In Canada, those rights have more teeth, and that alone produces much higher unionization.
Wal-Mart closes stores when unionization gains momentum. Pizza Hut (hearsay) has an undocumented policy of "fire people who talk about unions." When you're pulling near minimum wage, losing your job over labor laws isn't very appealing. I believe Chick-fil-A put wage discussions as fireable in my paperwork, as have other tech companies.
Some of the people who need unions the most can't get them, as the Labor Board hasn't enough teeth and the public isn't very favorable toward unions.
In Canada, those rights have more teeth, and that alone produces much higher unionization.