The reason that the Hittite/Canaanite army did not immediately attack the Egyptian army was likely because the Hittite/Canaanite chariots were heavy-chariots designed for decisive strike (much like the later cataphracts of Parthia). The Egyptians likely were able to deploy in formation, while the less maneuverable Canaanite heavy chariots lumbered into position. Also, unleashing the heavy strike power of the Canaanite chariots would have been a mistake, as once unleashed, the heavy chariots cannot be swiftly recovered, so the Canaanite commander was likely holding his chariot force in reserve.
The Egyptians used light chariots with two men crew, relying on speed and maneuverability to Canaanite three men crew heavy chariots relying on shock. In the open plains of Meggido, it seems that the light chariots had advantage over the heavy chariots.