I’m not positive that this is true, but I think that because when Quirrell shook Harry’s hand in the Leakey Caldron he was not intending to hurt Harry. Also, when Harry and Quirrell were in front of the mirror near the end of the first book Quirrell tried to hurt Harry so the Love of Harry’s mother turned him to dust. In addition to that, In the first book, Voldemort’s goal was not to kill Harry, but to obtain a body. The only reason he ordered Quirrell to kill Harry in the first book was because Harry had the Philosophers/ Sorcerers stone and therefore was in the way. Anyway note that this is just what I think and not necessarily true.