If I am not mistaken, Sao Paulo metropolitan area has around 12 million people. As all big cities, SP has a nasty traffic...sometimes around 100km during rush hours. Happens that CGH is in the middle of the city and GRU is far from the center, so it is more easy to get to where you have to be (home, meetings, etc...) landing at CGH. As any business, airlines follow passenger demand (supply X demand) and this is why it is one of the busiest airports in SA.