I think the reasons many people don't ship internationally have been made pretty clear in this thread so far.
It's all about risk versus reward. I'm not saying it's THAT much riskier to ship internationally, but it definitely is to at least some degree. And I think many sellers just don't think it's worth the hassle. And then you have people that hear horror stories and are just scared to even try it. And if we're comparing a US sale to a Canada sale, you can guarantee the sale to Canada will (a) generally take longer to ship, and (b) cost more shipping to the buyer. And let's think about this --- if there is a small issue in shipping - let's say a little slight damage. The US buyer paid less shipping and may be less likely to complain or make a big deal of it. But if you're the Canadian buyer and you just paid $15-$20 for shipping, you may be a little more frustrated.
NA is different from Ebay ---- I will ship internationally on NA, but I typically prefer not to on ebay. Here's why:
Let's take a relatively common item (not anything super rare or niche, etc), like a CIB Zelda NES game in good condition.
If I'm the seller, there is enough demand in the US alone, that I don't really need to offer international shipping to expand my potential customer base. Even if I charge actual shipping charges, so I get the same amount either way, it makes more sense for me to sell to the US buyer because shipping will be quicker, tracking is faster, and there is simply less risk overall of a problem. The entire transaction will be done quicker and I can move on.
I often sell on ebay with BINs since I tend to price a little bit low and just sell quickly. So for me, the argument of "well, offering the items to more people should hypothetically increase bidding" doesn't apply as much.
I think for large sellers this is less of an issue, but for smaller sellers it may not make as much sense.