Pork fat behaves.
It provides cohesion without a specific flavor but offers richness and mouthfeel you want. So it both provides the proper texture and binding from a physical perspective without interfering with taste. If you want to cut the fat, you can substitute cooked white rice for about 1/3-1/2 of the pork fat. It behaves differently but acceptably close.
It's stable. It's a saturated fat so it is slower to go rancid. This is good for flavor, curing and health.
Some would argue that it's consistent. And while it will certainly vary as any natural product, probably less so in comparison to ducks where it's easier to find breed variation than among commercial pork.
Good recipes rely on being reproducible and pork fat offers that.
Duck fat doesn't offer those features. It's good in it's own right of course and you can use it if you want. You need to be prepared for the differences in workability and flavor.
Depending on your grinder, pork fat generally grinds up better than poultry fat, at least in my Kitchenaid attachment.
Try grinding the meat and fat separately and then combine the small clumps of meat with different ratios of duck/pork fat Cook off samples of each and MAKE NOTES so you can refer to your findings in the future.