I don't think that the City has a requirement that the crus were the size that have been provided, or that they couldn't have been bigger, or deeper. The building was submitted as a DP, to zoning. The zoning says there should be a continuous retail frontage. It also says that to build the most condo space in this location, a proportion has to provided as an air right parcel to the City, containing non-market housing.
The developer, and their architects, which I think is The Hong Kong office of PDP London, with MCM, determined how to do that. I would think that the hill, and the accessibility requirements in the building code contributed to the design. The podium facing Robson has office space as well on two upper floors. The non-market units face the lane, and occupy the lowest part of the building, which you would expect as they're the lower value residential part of the building.
As long as the non-market meets the proportion of total residential space, and can be subdivided as an air right parcel, as far as I can tell, the developer can locate it wherever they choose within the building. I suspect the retail space here may have been viewed as marginal, by the developer. They're apparently not averse to bigger retail spaces, they have a purely retail and office building proposal next door at
1394 Robson, with larger crus. It will be interesting to see if they build that, sell the site off, propose a different project, or leave the site undeveloped for now.