IMO the falling GDP per capita is a bit of a mystery, I've yet to read a serious analysis that I believed. It's probably a number of factors, and most people would have some anecdotes that point to one or more of those factors.
Personally, having worked in the Ottawa high tech scene for 30-ish years, I'd say that a big factor is that almost every serious young, ambitious, entrepreneur that I've met has moved to the US. Was that because they didn't like Canadian taxes, weather, anti-business culture, etc? Not sure, maybe all of the above.