Why won't any law office represent me against my employer if I'm in a union?

If you were injured on the job as a result of your job, I would represent you in a Workers Compensation claim. No attorney wants any claim unless and until you can articulate how you can prove clearly that the employer owed you a duty of care and breached that duty, If your union owed you a duty of care (or a fidiuciary duty) and breached that, you can sue the union for the money damages you sustained just from that breach of duty.