Unions are the most useful when you're dealing with monopolies. Where there is fierce competition in an industry, most workers probably feel that unions are not needed, as they can get a better paying job by switching companies within the same industry whenever they like.

So I wonder if the recent rise in union interest is a symptom of the fact that in the U.S. a lot of industries have been monopolized or oligopolized, and the union option is starting to look increasingly more appealing every year.