That's because it's too treacherous to venture outside our igloos 350 days of the year!
But really, it's just because there isn't much selection up here. I live in a city of 500k, we have two stores that sell comics...
One is actually decent, but only sells new releases, once they're gone they're gone, they don't deal in anything older.
The other is 1/3 comics, the rest dungeons and dragons, MTG, and those little war hammer guys. Again, they only deal with new stuff...
I'd say 90% of my collection is from the states
Once in a while I'll find another Canadian seller online or on eBay, but it's fairly rare.