Why didn't the temperature continue to rise after taking the prime rib out of the oven?

Once you removed the roast from the oven and placed it in the room, the temperature cannot continue to rise because the room temperature was not over 145 degrees certainly. However, the roast would continue cooking on the inside, and that is what you have to account for. Plus while it rests, the juices have a chance to regroup throughout the meat and not wind up on the cutting board.