Because the US legalized gay marriage. Too bad for them, or luckily for us, we legalized gay marriage in 2004. I’m a little disappointed that none of those people would actually be willing to move here: it would be amusing to see the looks on their faces when they realised that Canada is not the place to find what they’re looking for.
http://mikesbloggityblog.com/americans-are-moving-to-canada-because-of-us-gay-marriage-ruling/