Because, there was no other evidence of "Freedom Club" existence outside Unabomber bombs and his letters. But truth is, investigators don't look what they had as evidence, but what wasn't in evidences. And this case was more like wild mass guessing than hard evidence with a list of probably bombers.

After bomb attack on American Airline Flight 444 in late 1979, an FBI-led task force that included the ATF and U.S. Postal Inspection Service was formed to investigate the “UNABOM”, code-named for the UNiversity and Airline BOMbing targets involved. One of first thing done by the team was analys of recovered bomb and looking on similar cases from the past. Bomb from airliner was similar to bombs used on attack on Northwestern University in May 25 1978 and May 1979, so working theory was this three attacks was done by same person or organization, hence they call case "UNABOM" not "Unabomber" which was popularized by media after Kaczynski arrest.

In 1980, profiler Chief Agent John Douglas with his Behavioral Science Unit in Quantico (working on this case is mentioned in his book "Mindhunter") create a bomber profile which discard idea of terror group behind this attacks and in his psychological profile describe him as a man with above-average intelligence and connections to Academia, neo-luddite and degree in hard science and being in his late teens-mid 20s during first attack, but due to lack of evidence it was discarded in 1983. FBI continue investigation looking for average blue-collar person with engineering related job like aircraft mechanic or electrican, possibly without job and living on somebody else money. As evidence of single person was very unusual for a terrorist group modus operandi, "bombers" don't issue their demand or political manifesto for years as first attack happened in 1978, all of them was done in similar pattern ie mail-bomb delivered to a single victim in his job, there was 6 years break in nearly random attacks which most of them happen on spring or fall and while his bombs do improve over years, they still follow same construction pattern and materials (especially hand-made wood boxes) as was made by same person.

FBI also does check out literally thousands tips all over the country looking for Freedom Club or Unabomber, yet they find nothing over years.

A final "trace" for a single person behind attacks attack was a bomb in front of computer store in California in 1987 where was a eye witness which provide Unabomber sketch. The only strong evidence not related to bombs which investigators had prior to his manifesto from 1995 and later securing his cabin in Montana.

FBI don't have any strong evidence for group existence, only evidences in case was his bombs, modus operandi, eye witness sketch and finally manifesto sent after 16 years since first attack with a couple letters. Enough to look for a one person over years and due to lack of any evidence which could suggest more than one person is responsible for this attacks, "Freedom Club existence theory" had to be discarded.