Obviously one reason is to reduce the number of people making D+ and Pinnacle by getting the bonus points in December. RCI knows that people at the Diamond and Diamond Plus level will still want to move up the ladder, double points or not, so they will just book more cruises to get there - more revenue and it also slows down the C&A program a bit.
I also suspect the point structure will be adjusted again and they might make D+ harder to achieve...perhaps make it 300 points instead of 175, and thereby make it closer to Pinnacle. This also plays into the ultimate plan of kicking everyone who doesn't have a gold card out of the Concierge Lounge once all ships are retrofitted with a Diamond Lounge.
I'm not that upset by this. I figured it wouldn't last, and got the hint last December when there were no special gifts, just a lapel pin and cheesy kitchen magnet.