Toronto was able to make the short list without any tax incentives.
Toronto has a better infrastructure than most US cities on the list, saner immigration policies, wages low enough (even if you compare the total cost of employment) to possibly make up for any tax discount. Canada's image is good among European and East Asian tech workers willing to relocate. And its tech scene, while not huge, is decently sized.
Jesus, Amazon is milking this for all it can and nearly every city is bending over backwards for Amazon.