The process of welding alone will heat up the thinner materials under 19mm enough to drive out the hydrogen to prevent hydrogen induced cracking, but the thicker materials act as a heat sink and quench the molten pool before all this takes place and cracking can occur at the root. The 3/4" (19mm) is just a place to start, they have figured out over many years of welding that 19mm and above need some extra heat to make sure all will go well, yet under 19mm no preheat is necessary.
John Wright