Waco Siege, Oklahoma City Bomber, North Hollywood shootout, Ruby Ridge. Columbine. Gang warfare.
Before "Terrorists" were dudes from the Middle East, "Terrorism" were religious cults, anti-government forces, and bank-robbers who had huge-ass weapons. Domestic terrorism was way scarier, even as Al-Qaeda bombed the World Trade Center the 1st time in the early 90s, the other domestic events overtook our mindset.
Now that Middle-East terrorism (ISIS, Al Qaeda, etc. etc.) seem to be on the decline (knock on wood), I'm not surprised to see the pendulum swing back to domestic terrorism.