Here is a quote from the first Harry Potter book, it's a conversation between Dumbledore and McGonagall.
"My dear Professor, surely a sensible person like yourself can call him by his name? All this 'You-Know-Who' nonsense - for eleven years I have been trying to persuade people to call him by his proper name: Voldemort." Professor McGonagall flinched, but Dumbledore, who was unsticking two lemon drops, seemed not to notice. "It all gets so confusing if we keep saying 'You-Know-Who.' I have never seen any reason to be frightened of saying Voldemort's name."
"I know you haven't," said Professor McGonagall, sounding half exasperated, half admiring. "But you're different. Everyone knows you're the only one You-Know- oh, all right, Voldemort, was frightened of."
So at this point Dumbledore says he has been trying to persuade people to call him his proper name : Voldemort.
But why wouldn't he try to persuade people to call him Tom Marvolo Riddle ? Certainly this name isn't as scary as Lord Voldemort is, and would encourage people to fight against him (because Dumbledore says something like fear of a name leads to the fear of the thing itself to Harry in the end of Philosopher Stone).
Furthermore, we see Dumbledore calling Voldemort Tom in his memories, when Voldemort applies for the DatDA teacher. And in OttP, during the Dumbledore-Voldemort duel, Dumbledore calls Voldemort Tom in front of Harry.
So does Harry during the final duel at Hogwarts, and Voldemort replies something like How dare you ? In my opinion Harry does so to show everyone else that Voldemort is just a man.
So, the question : Why would Dumbledore encourage people to call Voldemort Voldemort and not Tom Marvolo Riddle ?