Why are the pigs in charge of the meetings when all the animals are equal?

Animalism, the dominant ideology at Manor Farm, proclaims the complete equality of all animals. But, as the narrator wryly remarks, "Some animals are more equal than others." The pigs are the strongest and most intelligent of the farm animals, so they think of themselves as better able to lead and take control of the farm. And that is precisely what they do.
Unfortunately, the rule of the pigs soon degenerates into an outright dictatorship, with Napoleon calling the shots. Animalism has been cynically used and exploited as a cover for complete domination and control enacted by one single species. All animals were supposed to have been liberated by the new ideology, but in reality the pigs regard themselves as a class apart from all the others.
As the pigs are "more equal than others," they get special privileges such as milk and apples. Squealer, Napoleon's chief propagandist, tells the animals that this is because the pigs are more intelligent and so require extra rations to help them carry out their vital brainwork. In fact, says Squealer, pigs don't even particularly like milk and apples. But by consuming them, they are sacrificing themselves for the good of the farm.
All the supposedly cast-iron rules laid down when the animals took over the farm have been changed to suit the pigs. They get to sleep in beds; Napoleon starts helping himself to alcohol; and the prohibition against one animal killing another is completely disregarded in Napoleon's growing reign of terror. The suggestion is that no matter what noble, self-denying ordinances we impose upon ourselves in political society, the ideology of "might is right" will ultimately prevail.