I haven't got the full picture why they've chosen this format, except for making it shorter, but I do wonder, if it is anything to do with the amount of viewers, around the world that were watching the Associate games, especially when they played each other and even when they played one of the Test nations. Like many have said the ICC seems to run the game purely as a business entity, rather than whats best for the game and it may have come down to revenue streams, why they limited the WC to 10 teams. Which if it is the reason, is sad state of affairs for the governing body.
Personally, I think it would of been a excellent idea, to have something like home and away qualifying games for the Associates, in the lead up to the WC. It could be played, a year or two before the event and the best two qualify, like a competition within a competition. This would give the respective teams, something to plan for and give the players something to aim for, otherwise it gives a rather barren landscape, over the next few years for players both present and up and coming.