But the biggest reason is probably that its hard to make a skill-based game into something luck based.
Cockfighting is more of a luck-based game though your bird has a high chance of winning if it is well-trained but in the end luck should be on your side most of the time to win.
Is it though?
One rooster is not the same as another rooster. If anything that is what I imagine to be one of the most exciting parts of the game? To bet on a better rooster? Thats why in real life cockfighting the strong roosters are very prized. If you wish to emulate that experience and offer it as a game on a gambling platform, then you cannot make the digital version luck based. Otherwise it just becomes a simple dice game but with animated roosters...
Not that I am an expert in cockfighting. Far from it, actually.