It may be a question of price, though I also think that it could be a bout ease of buying. With the App Store people really only have to click twice to buy something. On Windows if you need a product as a solution to a problem, you need to go to different websites, compare their offers, everybody has a different payment provider, then you have to download and install. Really, I think that for many people pirating is not only cheaper but also far easier. And, since usually only good software is pirated, and not the crap, you don't even have to do so much research and cognitive burden to decide which offer to take. Other people already did that for you when they decided which one to pirate.

There're also studies though, where people find out that Mac users seem to be more inclined to pay for software. Maybe it's a question of initial price: When I pay a lot of money for a really good meal, I don't mind spending another 5 bucks for a great wine to accompany the taste of the meal. So if you spend quite some money for good (or perceived good, I don't want a Apple vs. others flamewar here) hardware, you might also want to spend a bit more on software.