I see nothing "illegal" or "prohibited" about this rifle design. It is not listed on the prohibited weapons list, there is no reason why you cannot legally buy it in canada.
Most of the time its supply and demand. If its really popular, it sells out in the states first, maybe a few reach us, but then we wait. The crap-shoot about living here and buying weapons made in the US is the ATF. The united states border patrol does not like to export weaponry, and it usually takes a long long time.
AR rifles right now from armalite are on a 4 month backlog becuase of border inspections, permits, etc for dealers.
I would talk to another dealer and ask them if they can get it for you. Where I live, pro-line shooters can apparently get anything you want, if you know the guys.
They were bringing in five-seven pistols from FN with 5.7mm ammunition a while back. As long as its not prohib you can buy it.