They also don't charge people who blow up abortion clinics with terrorism either. They haven't since the 60s - 70s.
If you look it up the courts have been petitioned several times to associate abortion clinic bombings with Christian terrorism but they keep refusing to call it what it is.
After reading about that fiasco I have very little faith our government actually has a working definition of terrorism that doesn't shift at their convenience.
it means that it needs to be an actual maintained organization, not Jim bob and his buddies threatening anybody they don't like. it's also not a requirement, it's only the reasoning provided.