DPR made extremely careless mistakes, too, to the point that even a random amateur investigator could've identified him, using only Google.
It's shocking how many of these people aren't caught sooner when they don't even know OPSEC 101.
No matter one's assumptions, it makes no sense to me that someone would register a VPS with their own information when it's pretty trivial to do so anonymously. Especially if you're running an illegal content hosting empire.
DPR's mistakes at least made sense to me; they're something anyone could have overlooked, even if they were still very naive mistakes. But I doubt DPR used his personal information when paying for servers. That's well beyond "unrealized mistake" into pure incomprehensibility.
A simple national security letter (NSL) without even needing to get a warrant and BOOM you can tap the server and get all info about the person running it.