It may be a year or two to go through the grievance procedure and get a case heard by an arbitrator. In the meantime, the member is waiting for justice. Arbitration is an expensive process both in actual money and staff time. These costs can quickly bankrupt a union treasury and employers know it. We also may not like what we hear.
Arbitrators can come up with a decision that can hurt the union.
Remember, when we take the case to arbitration, we must learn to live with the decision. The only real way to alter a bad decision is to sit down with the employer at contract time and negotiate better language, if that can even be done. Almost every local union we represent has a grievance procedure which ends in arbitration. Local unions should be using the grievance procedure to maximize advantage. That means that whole procedure from beginning, middle, and, if necessary the final step, arbitration.