Why dont they play at visy park but they do at aurora stadium?

Ok Visy Park holds 35,000 people where Aurora Stadium only holds 23,000 why not play at Visy Park. Is it because of Carltons contract why what is it.We have friends who go for Hawthorn and go to all games at Aurora and they says its a piece of crap. Why not play games at Visy Park when Carlton play teams like Adelaide or West Coast where they won't exceed 35,000 then play Collingwood and Hawthorn at th MCG I don't see why Visy Park is the best. Thanks.