As years pass violent right-wing (if we want to we can label those as McVeigh terrorists even if the FBI don't) anti- government individuals as McVeigh don't receive as much public notoriety press as people as Oswald and Manson.
Oswald, of course, killed a president and Manson was portrayed as a madman.
There is much more press value in there than in etching in the public mind ordinary far-right anti-government people. You know, we're told there are fine people in that group too.
Guessing there is a feeling that condemning those people, as say Manson was condemned, would intrude on the sensibilities of many other Americans. There are many who would see themselves as relating to them more.