Why All DDR's (DDR, DDR2, DDR3) internal clock sets to 200MHz
Because DDR is about the memory device's external interface, only and only about it.

Internally, SDR SDRAM, DDR SDRAM, DDR2 SDRAM, DDR3 SDRAM, and DDR4 SDRAM have (nearly) the same dynamic memory array built around capacitors and distinct in the silicon responsible for the interface (it consumes a significant part of the overall chip area) and in the volume of such the array.

As for up to today, the dynamic memory devices evolved in interface throughput (Gbps), manufacturing process (nm), and power consumption (volts), but not in the speed of the internal array, i.e the memory itself, which is kept (nearly) the same beginning from SDR SDRAM.