We found that, unlike in the U.S., our STEM-educated immigrants were not having as large an impact on innovation in the places they were moving to. We tried to dig a little deeper to find out why. The biggest piece of evidence we found was that, as you point out, only about a third of our STEM-educated immigrants are actually working in STEM. I think that's about half the rate in the U.S.
You asked why, and I don't have a good answer, because that was the extent of our study. I think there needs to be a follow-up study of that, but it's consistent with the story, or the cliché, that immigrant engineers are driving taxis in Toronto. Unfortunately, I can't tell you why, or not yet.