Sugar, while very valuable, is not necessary for survival. Sugar can be obtained from a wide variety of sources, including honey and fruits, whereas salt can only be obtained either from ground sources, which requires expensive transportation, or from evaporating seawater, a extremely time and labour-intensive process. Sugar was a luxury product that was eaten by the rich, whereas salt was essential for food preservation, particularly of meats. Remember, this is a time before refrigeration, so being able to salt something to make it last longer is critical. Also, salt is a necessary nutrient for survival, sugar is not. TL;DR: You need salt, both for nutritional purposes and food preservation, sugar was a luxury good that had a wide variety of potential sources and isn't necessary for survival.