The Unabomber was the target of what is still the most expensive criminal investigation of all time, and they still had absolutely *nothing* on him. They were looking in the wrong part of the country, for a totally different profile of person, and the few leads they were actually working on were red herrings.
He got his manifesto published, and used the phrase "eat your cake and still have it" rather than the more commonly-known version. His brother David just so happened to read that manifesto and remember how Ted used that phrase. David decides, on a lark, to go through some of the stuff Ted had left at their mom's house and finds an early draft of the manifesto. David, after much soul-searching, decides to report this to the FBI and they almost throw the lead out before deciding to actually investigate it.
Anything at all in that chain doesn't happen, Ted uses a different phrase, David doesn't read the manifesto or doesn't bother investigating Ted's old stuff, or Ted doesn't leave the draft in Mom's house, or David doesn't tell the FBI, or the FBI toss the lead entirely, and The Unabomber probably stays active to this day.