It has to do with business being efficient and competitive in a market system that is set up so that the consumer is the winner,.

There was a time in our history when unions played an essential role in insuring the employee was treated ethically and worked in an environment that was safe. Today, we have more government agencies in place to insure these things than any country on the planet. Today's unions benefit the few at the expense of the many.

Unions are historically most common in the manufacturing segment, and as US manufacturers have failed to be competitive with global competitors, manufacturing is less pervasive in the US-- I believe this fact plays a bigger role in the diminished number of unions (along with the fact that they are no longer necessary).