I mean it took them a really REALLY long to find ted kaczynski, while he was still bombing and being active.
And knowing it was most likely personal, i belive it was a one-off from him and the assasin wouldn't kill anyone else again and just stay low, and eventually the FBI would just give up on the case.
For all we know he could already be out of the US, i mean, i know it sounds stupid, given he's wanted, but knowing that there's no evidence on him save for a few blurry images he can put on a , wig, different set of clothes and just pass airport security and board a plane as if he was a regular joe, and proceed to hide in another country, at which point it'll become an international search operation which not many country care about, like, what are the chances some asian/african shithole is going to actively spend money they don't have searching some american guy who shot some other rich guy, ofcourse, this, and with no guarantee that he might even bein said african/asian country, so basically a goose chase.