Because only Order-of-the-Phoenix types used Voldemort's name:
“Exactly! You’ve got to give them credit, it makes sense. It was
only people who were serious about standing up to him, like Dumbledore, who ever dared use it. Now they’ve put a Taboo on it, anyone who says it is trackable — quick-and-easy way to find Order members!
Unlike the name "Harry Potter" which was used by many other people. (Harry and Potter are both incredibly common names in Britain). Furthermore, even assuming there was only one Harry Potter, there were still many innocent wizards who would use it, unlike "Voldemort", which only brave people used.
Even many order members were scared to use it:
‘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 sherbet lemons, seemed
not to notice.
‘It all gets so confusing if we keep saying “YouKnow-Who”.’ I have never seen any reason to be frightened of
saying Voldemort’s name.’
