How and why is it that the pigs gradually become the new leaders

The pigs represent leaders, specifically Soviet/Stalinist leaders. I think the negative connotations of pigs come into play here. They want everything, are greedy....Also pigs are known to be intelligent creatures. They use their intelligence to manipulate the masses (other animals). Orwell decides to give the other animals different traits as found in communist, and to an extent democratic, society.