From my own experience, there are a number of factors at play.
1) Labour costs are very local, and in Alberta they are significantly higher than many other places in the world
2) Material costs, especially on design-build jobs, are out of your control so labour costs are one of the few ways to manage costs
3) More labour doesn't always translate 1:1 to more production. So you could put more bodies on a task, double your labour costs, but only increase production 1.5x, in effect losing you money
4) Urban environments, especially older ones, have underground utilities that look like someone upended a bowl of cooked spaghetti onto your drawings. Add to that as-builts of those lines often being missing or flat out wrong, and you spend a lot more time on mitigation around them
5) Governments here will often have very strict rules about minimizing disruptions to road traffic and local residents (for good reason) that add complexity to scheduling that can really handcuff you on certain critical pieces of infrastructure.