I think it's pretty clear. This was one of the largest and easiest to use file-locker services. I used it tons of times because it was so convenient and the free service was much better than any of the other file-locker services. If you had an ad-blocker installed, the site was almost too good to be true. You could download huge files after a 30 second wait, at reasonably high speeds, no captcha or any other hoops to jump through, and you could download another file from the site immediately afterwards without wait (most sites make free users wait between downloads). Files would stay up on the site seemingly indefinitely, while other file-locker services would delete a file after it was not accessed for a specific number of days.
Also Kim was a larger-than-life target. He was living extravagantly. Right after he was arrested there was a huge Ars article about how ridiculous his lifestyle and persona was. So now the government could say: "Hey look at this asshole, he's getting rich off of piracy and stealing American jobs." They couldn't have hoped for a better target.
Unfortunately, Kim was obviously thumbing his nose at US copyright law and he thought he was untouchable because of his location. That might be a workable strategy for someone keeping a lower-profile but if you piss off enough powerful people they will find a way to get at you, legal or not. (As we are seeing here.)
That being said, since the guy had a family he really should have found a more respectable way to make a living. This was pretty reckless and selfish on his part.