Because DS piracy was done with a modchip. With the PSP, it was as simple as cutting a strip in the battery and downloading software (if a friend already did this, all you really need is to put their battery in your PSP, download the software, then give them back their battery). Sony did try revising the PSP to make it harder (it took about a year or 2 for the PSP-3000 to get hacked), but even those got hacked. You didn't actually need to buy anything to do it (though some sites already sold the modded "Pandora" batteries for those who didn't want to risk ruining their battery, which was possible if you messed up).