Because bystanders were not deliberately targeted by the pager attack. They were collateral. If I launch 20 rockets at a military base and a couple of them go astray and several civilians are killed, that is not a terrorist attack, even if I was aware of the possibility of that happening. Should I have been more careful? Maybe, but that is a different discussion. If I launch those rockets at a marketplace with the intention of killing/terrorizing civilians, that would be a terrorist attack. And that is the difference between the pager attack and Oct 7: intent.
War sucks. Doesn’t make it a terrorist attack.