I am quite sure as the article said that Right to Work plays into these numbers but some might just be lack of a population such as in my state because as a reminder, we are not a right to work state here in New Mexico. We just don't have a whole lot of people here. You can very much see where Right to Work has done it's damage.

About 14.5 million workers belonged to unions in 2013, accounting for 11.3 percent of the entire workforce.

The concentration of union workers varies largely across the country, with statistics indicating some states home to far more union members than others. Factors influencing the presence of labor unions include the types of industries employing workers and individual states' right-to-work protections.

The following table lists historical annual average shares of union employees as a percentage of a state's total workforce, according to federal data.
Union Membership State Statistics

And here is a map of the Right to Work states:

Map of Right to Work States - Political Ruminations