is the question more so why Canada does so many high-rise, concrete form apartments while the US sees "5 over 1s" dominate with large-floorplate, midrise apartment blocks?
There is a whole bunch of answers to that question, really.
1. US code requirements on midrise wood structures are less stringent. US code requirements in general are a lot less stringent than in Canada (generally - obviously it varies significantly across the US), at least anecdotally from what I have seen professionally and even on a personal basis just spending time in houses in both countries. Both countries use the same "parts bin" but have surprisingly different rules on how those parts are used.
2. Canada has spent 70 years perfecting the art of the repeating fly-form concrete highrise making it very cost competitive with wood. A lot of developers investigate doing wood-form midrise but often just do concrete as it's simply cheaper here. The construction industry knows how to do concrete and how to do it in a cost-effective manner - they don't know how to do wood-form.
3. Canada has much higher proportion of the housing market in apartments, meaning it needs far more units
4. Canada has much, much more restrictive zoning. The majority of homebuilding happens in areas with regulations rivaling the worst areas of the US on regulatory burden. Meaning that to pay to get your project through approvals you need a big project to spread those costs around
5. Did I mention Canada has more restrictive zoning? The US-style 5-over-1 needs large parcels of land to work well and Canada simply doesn't have large supplies of that kind of development land. What land we do have is expensive to entitle and in limited supply so developers do high-density.
Zoning and industry inertia are probably the biggest reasons.
Building codes are also a reason - 5-over-1s in Canada are hard to do from my understanding because Canada has much more stringent regulations on fire-separations (need to be heavier duty and be placed more frequently) which make very-large floorplate wood apartments cost prohibitive to the point where it's just cheaper to do a 20-30 storey concrete fly-form building than a 6-storey wood midrise.
That said, wood-form apartments are becoming more common in Canada again. They just don't dominate the industry like they do in the US. Places like
Squamish are doing decent amounts of wood-form apartments, and they are becoming popular in more mid-density Surburban areas in Ontario too.