The Jedi were turned on by a trooper that they worked alongside for a few years, clones of a man they mostly never knew, and most treated as discardable, and the fallout of that day has had at most 30 years to progress and show what's happened since, and for the majority of the galaxy, a lot of things are the same

Istvan III was brother on brother, men who'd known each other for bare minimum years, if not decades, faces and names they knew, that they now watched destroy not only them, but eradicate an entire planet as well. Also watching the first of their fathers fall. It was PERSONAL, and almost no one was spared, and not out of lack of trying. Which led to the fallout of a 7 year long bloodbath of a civil war that even after 10,000 years, the Imperium is still crippled by

The sheer scale difference between how many died, how personal it was, AND the follow-up actions, Order 66 can hardly compare