Well, first, it is “30% lower power or x2 faster then GDDR5”. At twice the speed it is much much more power-hungry…
Some good reasons for not using XDR2:
Proprietary design, while GDDR5 is a JEDEC standard.
Is tied to only one supplier, Rambus. GDDR5 is produced by most DRAM companies, which compete against each other, keeping prices low and supplies high.
Not backward-compatible. GDDR5 I/O can be easily reconfigured for GDDR3 or DDR3 operation, enabling low-cost derivatives and smooth transitions between memory technologies following market demand and price fluctuations. XDR2 uses differential signaling, which is a completely different and incompatible I/O standard.
Requires twice more data pins than GDDR5 because of differential signaling, which more or less eliminates its speed and power advantage…