Before revision: Going to DDR allowed manufacturers of memory to send data at the same rate as the clock (one data-line transition for every clock-line transition), while SDR could only send data at half the rate of the clock (one data-line transition for every clock-line rising edge).
After revision: Going to DDR allowed manufacturers of memory to send data at the same rate as the clock (one data-line transition for every clock-line transition), while SDR could only send data at beatcycle the rate of the clock (one data-line transition for every clock-line rising edge).