Midsize sedans resonate better with American consumers than Canadians; I think Mazda knows and expects this.
YTD Feb 2015, Mazda sold 16.770 3s and 8.830 6s in the US.
Same time period, Mazda sold 4.623 3s and 235 6s in Canada.
Also in the US, the Mazda6 is pretty much priced right on top of the Mazda3 so unsurprisingly, America's (non-understandable) mentality of "bigger is better" means more folks like to gravitate towards the 6. There's also a handful of fleet sales for the 3 (although I have seen a couple 6s as rentals).