At what point did western society decide it was standard convention to eat the sweet courses of a meal after the savoury courses and was there a reason for it. Other than the obvious savoury courses are, usually, healthier than sweet so it makes sense to eat the more nutritious food first when your stomach is less full.
Sugar was a late introduction to the already partially codified royal cuisines of France and Europe because of the high costs of import.
Rather than upend the order, desserts were added to the end of the meal where their flamboyant possibilities in flavor as well as sculpture could also provide a suitable climax to the narrative of an orchestrated meal. Sugar also paired well with the liquers, cheeses, and fruits that were served at the end of meals.
China, Japan, and other eastern cuisines made use of sugar more as a spice to modulate flavors in the savory side of the menu. These cuisines tended to have access to cheaper sugar earlier in their development.
Marie-Antoine Careme was one the first Chefs in France to feature sugar in candies and sculpture, at the Royal table. His recipes and sketches of centerpieces are widely available and well worth a study for his role in the development of cuisine in general.