I think there is an element of population/business density as well. Not that I have a clue what the distribution of car share members actually is I would guess that it mirrors the density patterns of the urban areas. Despite the growth in the Western Communities they are still, IMO, distinctly suburban. Given the design of the western communities, part-time ownership of a car seems impractical. There may be some diehards who can function without regular vehicle access but that number must be far lower than in the core. I mean, taking the bus to Westshore Mall is inconvenient as hell, I could not imagine trying to use public transport to commute.
Look at how crazy that Sooke bus is every morning, tons of married halfs that have no car.