We need to expand the set of possible actors and goals. The anthrax campaign happened immediately after 9/11. The result of the campaign was the end of effective political opposition to the massive expansion of military and "secret" aspects of the USA government made possible by 9/11. Cui bono? Certainly not the several implausible patsies the authorities immediately had lined up to blame.
Numerous USA calamities make a lot more sense when we study them with an eye toward who benefits rather than who is blamed.
What made 9/11 such a successful terrorist attack was that it made the US live in fear. That is a change (and it hasn't changed back). Terror (fear) is a perfectly fine end in itself for terrorism.
The anthrax mailing caused changes and fear in US society with very little work. That sounds like a very efficient terrorist attack IMO.
From this point of view, mailing of anthrax in 2001 was very inefficient.