The DPX2 is hard to fit a full bottle cage on as it has the Piggyback on it, supposed to far better for long gnarly descents to stop the can overheating. I doubt they would stock fit that with that design flaw. It's a fair bit more expensive than the DPS so over the course over however many bikes they sell that's quite a saving for them..
Very happy with mine..I use a bladder not a bottle cage which is the only drawback but you can fit a kids one in there.
Use this, problem solved and double the water storage ?