So-called "Right to Work" laws mean that a union-negotiated contract may be made available to non-union members. At that point, the individually rational action is to leave the union, keep the dues, but benefit from the union's negotiation. As everybody does this, the union declines.

"At-will employment" laws mean that the burden of proof for the cause of firing is on the employee, rather than the employer. This gives free rein to employer's to fire workers considering unionizing, so long as there is the barest pretext of another cause. As forming a union becomes economically risky, unions decline, or fail to form in new industries.

Unions have been attacked in the US, through explicit policies meant to defund unions, and to prevent unions from forming.