It takes more synthetic fill by weight to generate a bag rated the same as a down bag. Syntheticbags are only slightly heavier in summer and three season bags as less insulation is needed. Down stuffs smaller than synthetics. On an expedition this can save a lot of room in a pack. Down lasts longer than synthetics.
Synthetic fibers break down and loose loft much quicker than down.
2nd guide brought a synthetic bag on two 21 day Denali expeditions. By the end of the second over half the loft was gone. We have several people who have used one down bag on more than 10 expeditions without significant loss of loft. Synthetic bags are much cheaper than down. Synthetic bags are warmer when wet.