You either take below market rate (like myself) or you work in something like one of the 300 ad-tech or we'll-build-your-website/app dev mills (more often than not it's both).
There are a limited number of opportunities for truly interesting or innovative work if you don't want to work for "we're changing the world by creating the next uber/airbnb/cryptocurrency/coupon app!"
There are some seriously good and interesting companies here, but many exist out of the popular eye and/or usually located in the suburbs. (at least around Toronto)
But mainly I suspect it's the pay and career trajectory. Get a gig at FAANG? You can write your ticket after that. Work at a major Canadian company? You won't get a call back from other Canadian companies at a pay cut...