Because Amazon Canada's selection is pretty terrible compared to Amazon USA.
Amazon Canada is the whole reason why Canadian online shopping is such a terrible experience.
First off - Amazon Canada is NO CHEAPER than retail. I'd find stuff cheaper at Future Shop/Best Buy than at Amazon. Most Canadian retailers are like that - online prices generally aren't great - if you're savvy, you can find it available at the brick and mortar cheaper and available right there. And, save the shipping since few Canadian retailers other than the big guys (Future Shop/Best Buy, Amazon, Walmart, etc) offer free shipping.
Amazon.ca does have some stuff that is cheaper. The problem is that Amazon.ca has next to nothing in comparison to Amazon.com. For example, my sister bought a high end ASUS laptop a few years back for about $200 cheaper on Amazon.ca than it was on Amazon.com. Also, my Dad bought some replacement batteries for his portable battery packs with free shipping for a better price than he could get them anywhere local and for a price similar to the Amazon.com prices. For the most part, though, it seems that Amazon.ca is a placeholder. It doesn't look like Amazon is serious yet about growing in Canada.