Thaumagurist*Dirk Gently They used to rotate them around teh continents but decided a few years ago to scrap that. So now every WC is open to bidders from anywhere.
Was there a reason why they decided to scrap the rotation?
Not 100% sure but I think it was to do with Blatter's long-stated desire to get a WC to Africa. If there was a strict rotation then there wasn't flexibility to take the WC there, and haiving scrapped the rotation to have the WC in South Africa no-one really wanted to put it in place again.
To be honest, though, I'm not sure how formal it ever was - more like a gentlemen's agreement or they way they'd always done things, rathern than anything written into the FIFA rules. So much of the WC decision-making is done behind the scenes with deals between counties to stand aside for other bid in return for future support, etc.