The most embarassing thisg possible for TSA would be an exact repeat of 9/11 - same weapons (wasn't it box cutters?) etc. So that's what they target first.
A new kind of attack is harder to predict and easier for the authorities to explain by saying "nothing like this has ever happened before, there was no way we could have prepared for it." And there's no way they could cover all possibilities anyway.
And to be fair, copycat attacks do happen (e.g. July 21, 2005 in London) so it is not a total waste.