There are many ways that this has happened. Let us look at two of them.
First, many more American jobs today are white collar jobs. These jobs tend not to be unionized or, really, to need unionization to any great extent. This has weakened unions overall in the country.
Second, a major change in the workplace is the need to compete with countries where people will work for much lower wages. This need has led to difficulties in labor relations. Unions are understandably reluctant to accept wage cuts but companies feel that they need to make those cuts if they are to keep jobs in the United States.
For reasons such as these, labor relations have changed and the influence of labor unions has declined.