Exactly why do many people confused Link's name with Princess Zelda's name? 全体, 全体的です confused Link with Zelda?

The reason for this is because, back when Legend of Zelda came out on the NES, not everyone knew who Link was. And since a characters name in the タイトル was Zelda, everyone assumed that Link's name was Zelda. Even I thought this when I first played the game on the Legend of Zelda: Collectors Edition for Gamecube

Pretty much what Windwakerguy said.

But at this point, it has pretty much became a worn out running gag of the fandom.