This kind of collective hand wringing always gives me a chuckle. I'm from Vancouver and moved down to the valley in 1998. My rationale:

  - Vancouver has a high cost of living without commensurate salaries (and often no equity).
  - Because of the wealth of technology companies, it's easy to find a job here.
  - The weather (at least in the valley) is far better than sitting in the rain (Vancouver), or snow (rest of the country).
  - Quasi-reasonable immigration policy (TN-1 visas are easy to get, although have weird drawbacks)
  - A common language, and a fairly similar culture.
  - Proximity to home (it's only a 2 1/4 hour flight back to Vancouver with no time change -- can someone start flying out of SJC instead of only SFO?).

The start-up culture really fuels the tech industry here. Since moving in '98, I've been through two IPOs where both companies ended up having billion+ dollar valuations. I think the reason why it's so successful is because of reasons I (and others) have already mentioned, but also because of:

  - decent universities
  - former military/government technology jobs
  - easy access to capital

and probably most importantly, past success. The Bay Area has this unique ecosystem where companies have been bootstrapped from nothing except an idea, and that gives other people confidence to try their own hand at a startup. This feedback loop is what keeps the valley humming.

After having been so successful being an employee at two startups that did well, I started my own company a couple of weeks ago. It's absolutely amazing the amount of support you get here, and the network of people who you can tap to ask questions and help out in your venture.

Vancouver and Toronto both have parts of the equation, but they don't have the whole package. And neither of them have the kind of past success which breeds future success.