Not to take either side in this, but I can actually see a way that piracy would have a greater affect on PSP than on DS.
Because the PSP has functions other than as a gaming device, it's possible a lot of people choose to purchase it specifically for pirating games, as well as other content. It presents a lot of ease of use, since you can have your games, your music and your movies all on the same device. So you have a contingent of people who might be buying the PSP specifically with piracy, on all levels, in mind. Then you might have people who buy it for the media functions, and then, because it's so easy to pirate games, they go ahead and download a few games onto a memory card as well. (This is sort of a backfire of what Sony originally intended, convince people to buy it as a media player, and since they have the device, they might as well buy games for it, too...but, since it's so easy to pirate, why should I buy these games?)
Whereas with the DS, it has such a broad demographic, the people who actually purchase a DS with the intention to pirate games is only a small portion of that demographic. Then you have an additional portion that will come across the means to pirate and run with it.
Of course, this doesn't explain Stumpokapow's central point, which is why this would affect lower tier games on the PSP less than it's affecting higher tier games. The thing is, we don't know that it isn't. It's entirely possible that MHP2 would have sold 3+ million by now if not for piracy.
On the other hand, one of the things about easy piracy is that people who otherwise wouldn't be interested in something (for instance, people who bought psp as a device other than for games) might be willing to try that something if they can do so for free. So it's entirely possible there are a lot of PSP owners who aren't gamers, and generally don't care for games, who pirate PSP games just because it is so easy. This is less likely to be the case with the DS simply because it doesn't really do anything but play games.
