I'm simply not a fan of any business building huge HQ offices in Toronto. Ontario as a province is all too happy to regulate the crap out of things that don't need it, and the population has an amazing capacity to support the regulations, regardless of merit, as long as they are different enough from whatever the US is doing.
The weather is terrible compared to Vancouver or ANYWHERE IN THE US. Yeah, it's easy to bring in immigrants, but that's not enough for a 50,000 strong hq.
Let's not forget that they are in the midst of a huge, epic housing bubble that dwarfs anything seen in the US in 2007. Toronto is insanely expensive for the privilege of living in a city whose weather is approximately (less snow, but still) like that of the cities in the US that are known for their terrible winters. (Buffalo, Erie, Cleveland)