Why use DDR instead of increasing clock speed?
With SDR, there are two clock edges per bit, but only at most one edge on the data line.

With high frequency communication, the analog bandwidth limits how close you can put edges together on any given wire. If your clock signal hits that limit, you're wasting half of the bandwidth of the data wires.

Therefore, DDR was invented so that all of the wires hit their bandwidth limit at the same bit rate.