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. When very wet, down is all but worthless. Most of us have spent cold nights inside wet down coats or bags- a miserable way to spend a night. Don't do it. Even in a wet environment like South Central Alaska in the summer, either insulation can do fine if you are sleeping in a tent.