The reasons are well known, Canada does not have enough population for a real megacity, and Vancouver doesn't have enough flat land to expand on without developing the Valley, some of the best agricultural land in Canada. The Fraser Valley, outlined in red on this map is about 70 miles/112 km from the east to the west end of Vancouver, there is nothing but mountains to the north (and farther east), ocean to the west, and the US border to the south. In spite of this, the city metro will realistically still increase probably to 5 mil in the not so distant future.
Having the mountainous scenery and coastal areas of BC for outdoor adventure, and the mildest winter climate of any major city in Canada certainly helps create demand to live here. Canada's political and economic stability encourage immigration and investment, but it would take much more to be anywhere near a mega city.
Here's a new song, by Australian singer Lach Marshall, entitled Vancouver.
Lach Marshall - Vancouver
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