The chief problem is that construction workers have no job security. Their work is temporary, they go from job to job, from employer to employer. When they are working at one site, they are already wondering where the next job will come from. Every construction union contract that I know of gives the employer the unqualified right to reject any application for a job, even when they are referred from the union hiring hall. There is no recourse against unfair treatment.
Some contracts even give the employer the right to hire and fire without qualification.
Employers and union officials are both content with this setup. For the employer, if he keeps the business agent happy, he is free to hire and fire whom he pleases and to create not simply a satisfactory workforce, but a docile one which will not complain. For the crooked or authoritarian union official the system permits him to build a political machine, to reward friends and punish critics, to discipline or starve out those who would oppose him. This system makes a mockery of any grievance procedure. A worker who files even justified complaints will soon find that all doors are closed.