From what I understand Usenet is a more distributed system, with usenet servers sharing vast amounts of data between each other. Many of those servers are not located in the US, so prosecution becomes difficult without international cooperation.

It’s also a system that’s been around since 1980 and hasn’t been taken down yet. I’m sure individuals providers have overtime, but usenet as a whole chugs on with data exceeding 20 years old depending on the retention of your particular provider, usually 5000-8000days.