Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_5 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8L1 Safari/6533.18.5)
Because when apple released the current Mac Pro the iMac didn't either. Since then a new model iMac has been released they updated the options. Apple just hasn't bothered updating the Mac Pro page. I 100% expect it to be an option when the new Mac Pros are released. (whenever that is lol)