"Why is that?"

Roosters are lean. The meat is tough. That's why you find typical rooster recipes are boiling or simmering for a long time like chicken and dumplings or Coq au Vin.

Gyoza benefit from the texture of that tough meat. If you grind it you remove that texture. It's also lean with very little fat. Ground meat loves fat. But by mincing it you get all the flavor without adding fat. You get a satisfying flavor and toothiness.

We have a general "no roosters rule" but sometimes they are unavoidable. We let them live their best life until they become a danger to the hens or my wife is tired of getting attacked first thing in the morning when she lets them out.

That's when they go to freezer camp. With gyoza I can make a big batch from one rooster. One of the other issues with roosters is that despite their size they don't have a lot of meat on them. So you might get one meal from the other popular recipes. But with gyoza you are bumping up the meal value with cabbage and other things. You can get 40 gyoza from a single bird. And you don't need to eat them all at once.

I make the gyoza and place them on a baking sheet and freeze them before tossing them in a freezer bag. Now I have quick fix meals for the future. I can deep fry, air fry, steam or pan fry them and have a quick meal after a long day.

So that's why gyoza and why minced instead of ground.