Because Voldemort was gone, but by no means forgotten.
For a long time, Voldemort was the monster under your bed, the thing that went bump in the night, and the monster that gave nightmares to your nightmares. It's a bit hackneyed to say, but he was the Nazi Germany of the wizarding world - when active, he was the looming monster that everyone not on his side feared. When defeated, he was the specter that loomed in recent memory.
Even after his defeat, no body was ever found. No one could be certain he hadn't simply been pushed back, to return eventually.
Every witch and wizard who hadn't been a Death Eater (and many who had) feared his return. When he was alive and active, he could likely have used magic tied to his name (similar to the taboo spell in book 7, though obviously not the same since he didn't control the Ministry). There were likely other, similar spells (on a smaller scale) and Voldemort could have used those to target people, especially in the early days. It's the only reason I can come up with for people not using the name, instead calling him 'You-Know-Who'.
This habit would have been hard to break after his defeat, especially if you were afraid he might not be gone for good.
Also, recall that the Death Eaters still outside of prison had all betrayed Voldemort - they'd either denied being with him ("mind control, I swear!") or sold out loyal followers to avoid prison themselves - they would want to ignore his attention as much or more than normal wizards.
Finally, your point about Aurors: Dumbledore was the only wizard Voldemort feared. He was Voldemort's ONLY peer. Moody, one of the best Aurors, could not have gone toe-to-toe with Voldemort. Maybe several Aurors, set up in ambush, catching Voldemort in the shower would have stood a chance...but I doubt it. Voldemort was intelligent, clever, and powerful. He also lacked anything even resembling morality. No one would cross him if they could avoid it, save for Dumbledore.
