I think the real reason is: when downtown Vancouver and downtown Toronto were being developed in the 60's and 70's the focus was on multi-level malls. On the other hand, the newer suburb areas have "power centers" for a couple of reasons:
1) Retailers want to be able to open for extended hours if they so please without having to pay the overhead associated with security and overhead of an entire mall. They also want to have parking in close vicinity to their store.
2) Consumers have shown that they prefer the power center concept where they can just drive up to their selected store without wandering through a mall. This is evidenced by all the dead, dying and demolished malls in North America. I don't think it is a good idea to try to force malls on people who don't want them. However, since the younger generation doesn't seem to drive as much, maybe that will change?
Here are a couple aerial map links to power centers in Mississauga -
https://www.google.ca/maps/@43.59841.../data=!3m1!1e3 and
https://www.google.ca/maps/@43.61440.../data=!3m1!1e3. The second Mississauga example dwarfs Dartmouth Crossings -
https://www.google.ca/maps/@44.70503.../data=!3m1!1e3. The Mississauga example is a mix of industrial and expansive commercial/retail (so if you include Burnside with Dartmouth Crossing they might be comparable). It may not be pretty, but it certainly is popular.
Well, you might be thinking that this is just Mississauga
; however, I can give examples of power centers in Milton, Oakville and Burlington; and these are just in the western suburbs of Toronto.
Here is a smaller one that is popping up in Hamilton -
https://www.google.ca/maps/@43.19494.../data=!3m1!1e3.
Just a positive note: consider Hamilton that was going through a period of slow growth due to reductions in the steel industry, its power center on the mountain looks rather small; isn't it somewhat encouraging that the Halifax Metro area can support the Dartmouth Crossing, which will get much larger once IKEA opens?
Let's face it, places like IKEA would not have located in the developed areas of central Halifax.
I could go on and on with examples, but I have failed in my attempt to keep this post concise