Because they were so intimitated by and scared of him and the horrors he caused, it was frightening to just speak. Ron Weasley saw it as a sign of "respect", even if you hated him. Voldemort demanded this respect, so many where too fearful to stand up to him (notable exceptions being Dumbledore, Harry, and some of the Order of the Phoenix.) It also possible they feared it would be detected through magic and force consequences, which turned out to be kind of true when they put a taboo detection curse on it at the end of the second war, capturing the Trio.
correction: All of the Order of the Phoenix, with the glaring exception of Peter Pettigrew, stood up to Voldemort.
It is confirmed that people were afraid to say his name out loud before the First Wizarding War, though what deeds he committed to gain this infamy are unknown. Its possible he's placed a taboo on his name in the past to instill fear and control.
If Voldemort had put a taboo on the name in the past, why wouldn't the oldtimers have remembered and shared that information?