Basically, the government made a contract, essentially a collective agreement, with the building trades of the area whereby it set the terms and conditions for all employment—wage rates, pension benefits, and all that. But in addition, only members of the building trades could work on the project, even though in Manitoba union density is even lower than here.
So it excluded 85% of the construction workforce of the province from working on the job, but it also excluded all the open-shop contractors who might have wanted to do the job.
Basically, it's eliminating the competition and giving the work to your friends.