The environment for labor unions today is skewed in favor of employers. Labor unions are rapidly losing bargaining power given today's economic and political climate.
Economically, the rise in white-collar jobs and increasing globalization have hurt unions. Workers whose jobs could be sent off to China at any moment are in no position to push for unionization and higher wages. Politically, the United States has become much more conservative. The rise in "Tea Party" sentiment in the country has led to tremendous reductions in support for labor unions in both the private sector and in government.
For these reasons, it is becoming much harder for unions to organize and for already-existing unions to maintain their power or make any further gains.