When Bruce Lee was doing research in order to create his Martial Art of Jeet Kune Do, he did extensive research into Western Boxing amongst other arts. One of the people he researched was the famous boxer Jack Dempsey. In his book Championship Fighting, Dempsey explained the importance of the lead hand jab and also of the vertical fist, the thumb faces up as opposed to facing the side like in a typical jab.
When throwing a punch with a vertical fist, the human arm is in a structurally stronger position. This is why in all of Bruce Lee's writings, the straight lead is thrown with a vertical fist.