To answer the original post: because if you treat every non-terrorist as a terrorist, you simply erode meaningful use of the word “terrorist.”
@marinelife is right, @squirbel. Terrorism is a tactic, one frequently employed by governments and militaries, to deliberately instill fear in people or the population, usually for ideological/political reasons. Maybe the Sikh temple shooter was doing that, but I’m not so sure. I think it’s more likely he was just acting out some right-wing fantasy about killing brown people or Muslims. Maybe he was trying to get revenge for something – which might elevate it to an act of terrorism.
The unambiguously accurate way to describe such cases is “mass murder,” not terrorism. The other recent mass murder in the news recently, in Aurora, almost certainly seems like it was not terrorism. It seemed design to entertain the mentally unstable shooter, and perhaps had little other purpose at all.
Terrorism doesn’t need to involve killing people either. IRA bombs were meant to destroy landmarks, and they largely tried to avoid killing people. The Arkansas cat killing, where the word “liberal” was scrawled onto the murdered cat of a political candidate, might count.