I honestly have no idea why this is so hard to prosecute? Why is this all done in civil court? I mean, most of it takes place in public. I don't exactly agree with the draconian sentences leveled out by current laws, but those draconian sentences are merely a sign of how weak and pathetic current enforcement is. As I've said before, cruelty is a sign of weakness. If we quickly identified offenders and levelled out proportionate fines on a massive scale, it would take little time for piracy to level off. As it is, in a realistic sense, is it really even illegal? If I have a piece of paper saying "This action is illegal and you'll die if you do it!", and no one on Earth backs it up or gives a fuck about it, what have I really accomplished? That's the current law against piracy. It talks the talk, it does not walk the walk. It is weak, and no one respects it.
I think it's hard to prosecute because it's hard to build a case/get to that point in the first place. Your isp doesn't know what you are specifically downloading, and even if they did, you could just say it was someone else on your wifi network (mine is locked but that's the excuse I would use in court )