The reason this is failing is because of awareness. I travel to Toronto for work 4-5 times every year (for the past 12 years). I heard about this extension being approved a couple years ago and I've never seen a sign for it in Pearson (there must be some somewhere). I've also asked people who live in Toronto whatever happened to this project and nobody even knows what I'm talking about.
Meanwhile, I learn from this article that it's actually up and running and they not only consider it a failure but they're using it to say other transit systems won't work. A Musk built project won't fail on awareness like this project has.