Because as you say there was probably already a contract or clause in a contract warranting a second sequel if the first made x amount of dollars ... and that despite the lower than expected profit of 'GF Returns', Random House must've placed bets (aka rolled the dice) that there are enough Godfather 'whores' out there who will be too tempted not to 'pre-order now'... whether or not they end up actually reading it.

Apple